Support compiling and testing with Java 21

[octylFractal]

Steps to complete

Beta Give feedback

1. Add JDK to CI (separate PR, so the below is testable)
2. Bump ASM version to 9.5 #25708
   +0
   
3. Remove unsupported warning from JavaVersion
4. Add future entry to JavaVersion to maintain 2-year window of future version constants
5. Update JaCoCoCoverage cutoffs if necessary
6. Update compatibility table
7. Update GroovyCompileToolchainIntegrationTest#can compile source and run tests using Java #javaVersion for Groovy, and the similar one for ScalaCompileToolchainIntegrationTest
8. Update JavaVersionSpec and JavaClassUtilTest
9. Update list of cross-compile JavaVersions in JavaCrossCompilationIntegrationTest

[StevenCurran]

What is the plan for this? Full 21 support in 8.4? How much longer after 21 GA until full gradle support?

[octylFractal]

We cannot give an exact timeline for full support, as seen in #25574 it is dependent on Kotlin.

Support for compiling and testing should land in 8.4.