Gradle 8.2 compatibility

[TWiStErRob]

Is your feature request related to a problem? Please describe.
Gradle 8.2 deprecated conventions with nagging. They were deprecated for a long time, but without nagging.

https://docs.gradle.org/8.2-rc-2/userguide/upgrading_version_8.html#deprecated_access_to_conventions

Describe the solution you'd like
Migrate away from using conventions to extensions, so builds can pass nicely even with org.gradle.warning.mode=fail. At Gradle 9.0 this will become an error, so sooner or later the migration has to be done. Better do it sooner :)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

The Project.getConvention() method has been deprecated. This is scheduled to be removed in Gradle 9.0. Consult the upgrading guide for further information: https://docs.gradle.org/8.2-rc-2/userguide/upgrading_version_8.html#deprecated_access_to_conventions
	at org.gradle.api.internal.project.DefaultProject.getConvention(DefaultProject.java:593)
	at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:64)
	at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:476)
	at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:475)
	at org.gradle.api.internal.provider.DefaultProvider.calculateOwnValue(DefaultProvider.java:72)

(this is just one example, you can see all usages at https://github.com/TWiStErRob/net.twisterrob.chroma/actions/runs/5355711906/jobs/9714328563?pr=13)

[TWiStErRob]

I had a quick look, it doesn't seem it needs a breaking change. In my simple project there are only a few usages, all centralized in the same method sourcePluginXmlFiles:

There's 5 usages of `org.gradle.api.plugins.JavaPluginConvention`

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:65)
at org.jetbrains.intellij.IntelliJPlugin.configureTestTasks(IntelliJPlugin.kt:1169)
at org.jetbrains.intellij.IntelliJPlugin.configureProjectAfterEvaluate(IntelliJPlugin.kt:387)
at org.jetbrains.intellij.IntelliJPlugin.access$configureProjectAfterEvaluate(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$7.execute(IntelliJPlugin.kt:236)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$7.execute(IntelliJPlugin.kt:235)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:65)
at org.jetbrains.intellij.IntelliJPlugin.prepareConventionMappingsForRunIdeTask(IntelliJPlugin.kt:888)
at org.jetbrains.intellij.IntelliJPlugin.access$prepareConventionMappingsForRunIdeTask(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$1.execute(IntelliJPlugin.kt:189)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$1.execute(IntelliJPlugin.kt:188)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:65)
at org.jetbrains.intellij.IntelliJPlugin.verifyJavaPluginDependency(IntelliJPlugin.kt:393)
at org.jetbrains.intellij.IntelliJPlugin.configurePluginDependencies(IntelliJPlugin.kt:1580)
at org.jetbrains.intellij.IntelliJPlugin.access$configurePluginDependencies(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureDependencies$ideaPlugins$1.execute(IntelliJPlugin.kt:308)
at org.jetbrains.intellij.IntelliJPlugin$configureDependencies$ideaPlugins$1.execute(IntelliJPlugin.kt:307)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:65)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:476)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:475)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:65)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:476)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:475)

There's 5 usages of `Project.getConvention()`

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:64)
at org.jetbrains.intellij.IntelliJPlugin.configureTestTasks(IntelliJPlugin.kt:1169)
at org.jetbrains.intellij.IntelliJPlugin.configureProjectAfterEvaluate(IntelliJPlugin.kt:387)
at org.jetbrains.intellij.IntelliJPlugin.access$configureProjectAfterEvaluate(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$7.execute(IntelliJPlugin.kt:236)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$7.execute(IntelliJPlugin.kt:235)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:64)
at org.jetbrains.intellij.IntelliJPlugin.prepareConventionMappingsForRunIdeTask(IntelliJPlugin.kt:888)
at org.jetbrains.intellij.IntelliJPlugin.access$prepareConventionMappingsForRunIdeTask(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$1.execute(IntelliJPlugin.kt:189)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$1.execute(IntelliJPlugin.kt:188)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:64)
at org.jetbrains.intellij.IntelliJPlugin.verifyJavaPluginDependency(IntelliJPlugin.kt:393)
at org.jetbrains.intellij.IntelliJPlugin.configurePluginDependencies(IntelliJPlugin.kt:1580)
at org.jetbrains.intellij.IntelliJPlugin.access$configurePluginDependencies(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureDependencies$ideaPlugins$1.execute(IntelliJPlugin.kt:308)
at org.jetbrains.intellij.IntelliJPlugin$configureDependencies$ideaPlugins$1.execute(IntelliJPlugin.kt:307)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:64)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:476)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:475)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:64)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:476)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:475)

There's a 4 usages of `org.gradle.api.plugins.Convention`

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:383)
at org.jetbrains.intellij.IntelliJPlugin.configureTestTasks(IntelliJPlugin.kt:1169)
at org.jetbrains.intellij.IntelliJPlugin.configureProjectAfterEvaluate(IntelliJPlugin.kt:387)
at org.jetbrains.intellij.IntelliJPlugin.access$configureProjectAfterEvaluate(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$7.execute(IntelliJPlugin.kt:236)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$7.execute(IntelliJPlugin.kt:235)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:383)
at org.jetbrains.intellij.IntelliJPlugin.prepareConventionMappingsForRunIdeTask(IntelliJPlugin.kt:888)
at org.jetbrains.intellij.IntelliJPlugin.access$prepareConventionMappingsForRunIdeTask(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$1.execute(IntelliJPlugin.kt:189)
at org.jetbrains.intellij.IntelliJPlugin$configureTasks$1.execute(IntelliJPlugin.kt:188)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:383)
at org.jetbrains.intellij.IntelliJPlugin.verifyJavaPluginDependency(IntelliJPlugin.kt:393)
at org.jetbrains.intellij.IntelliJPlugin.configurePluginDependencies(IntelliJPlugin.kt:1580)
at org.jetbrains.intellij.IntelliJPlugin.access$configurePluginDependencies(IntelliJPlugin.kt:122)
at org.jetbrains.intellij.IntelliJPlugin$configureDependencies$ideaPlugins$1.execute(IntelliJPlugin.kt:308)
at org.jetbrains.intellij.IntelliJPlugin$configureDependencies$ideaPlugins$1.execute(IntelliJPlugin.kt:307)

at org.jetbrains.intellij.Utils.sourcePluginXmlFiles(utils.kt:383)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:476)
at org.jetbrains.intellij.IntelliJPlugin$configurePatchPluginXmlTask$1$2.call(IntelliJPlugin.kt:475)

Raised #1416 to fix.