Gradle: Support gradle plugin dependencies using variables to specify the plugin version

[Jnosh]

What would you like Renovate to be able to do?
Renovate should update gradle plugins for which the version is defined in settings.gradle.kts using delegated variables set from gradle.properties.

Describe the solution you'd like
Renovate already handles variables for setting dependency versions in build.gradle.kts so the same should be supported for gradle plugins.

Describe alternatives you've considered
Don't implement this feature, require users to set plugin versions as strings instead of using variables.

Additional context
I added an example repository demonstrating this issue here: https://github.com/Jnosh/renovate-issue

As can be seen in the renovate master issue, renovate correctly detects the plugin dependencies and that they need to be updated but fails to create pull requests to perform the updates when the plugin version is specified using a variable.
Relevant settings.gradle.kts and gradle.properties.

Relevant code seems to be in lib/manager/gradle/build-gradle.ts.
If I interpret the code correctly, the issue seems to be that gradle plugins use a different dependency specification format that only includes the dependency group.

Renovate already has a matcher for detecting these dependencies (kotlinPluginStringVersionFormatMatch) but is missing a matcher to identify the version variables for this case to add them to the version variable map.

[renovate-release]

🎉 This issue has been resolved in version 19.223.0 🎉

The release is available on:

• GitHub release
• 19.223.0

Your semantic-release bot 📦🚀