Remove imports of old maven plugin classes for Gradle 7.0 build compat

[m8nmueller]

The 'maven' plugin which is deprecated for quite some time is finally removed in Gradle 7.0. This also affects the related classes (their replacements already live under org.gradle.api.publish.maven).

The ShadowJavaPlugin class currently imports them which makes it incompatible with Gradle 7 at compile time. By removing the imports and using Groovy's def variable declaration, the search for those old classes is deferred to runtime, only being called when the 'maven' plugin is applied and the classes are there as expected. So this change is compatible with use of the plugin in older Gradle versions.

As Gradle emits warnings for the jcenter repository, claiming it to be removed in Gradle 8, I followed their suggestion to change it to mavenCentral.

With this change, I am able to build and use this plugin with Gradle 7.
Fixes #658

[krolik-exe]

You Can just remove the jcenter() without replacement as GradlePluginPortal() has everything , see #659

[m8nmueller]

Thanks for pointing out, @krolik-exe!