Update workspace to Gradle 9 (doesn't affect build)

[Jonathing]

This PR updates MixinGradle's build environment to Gradle 9.2.0.

Here are some highlights:

• This PR does not make any changes to MixinGradle.
• The Forge maven hosts mirrors of a Gradle API redistribution by Remal (remal-gradle-plugins/gradle-api) so it can continue to target Gradle 6.6.
• Added a buildSrc which contains a slightly modified version of the Groovydoc task to allow it to be run under a forked JVM.
  • This allows Groovydoc to be run using JDK 8 despite the limitation of needing to use JDK 17 for Gradle 9.
• Fixed any buildscript issues that could cause issues with eager creation of DSL objects or the configuration cache.