[7.0] Begin ForgeGradle 7.0's Release Candidate lifecycle

[Jonathing]

The merging of this PR will mark the beginning of ForgeGradle 7's Release Candidate lifecycle. This crucially means a few things:

• API breaking changes will no longer be accepted.
  • All changes to API must strictly be additive until ForgeGradle 8.0.
• There might still be some bugs that need to be ironed out before the full release.
• As we get closer to full release, I will begin slowly phasing out ForgeGradle 6's development (though all versions of ForgeGradle are open to bug fixes and PRs).

As an additional note, I'd like to begin stabilizing the other plugins as well. This includes JarJar Gradle, Multi-Release Java, and JVM Aggregate Testing.