Declare the hosted Java selector for Forge 1.20.6

[SkyBlade1978]

Summary

• declare java_setup_version=21.0.7+6.0.LTS for GitHub's Temurin catalog
• retain java_toolchain_version=21.0.7+6 for the exact Gradle/runtime contract
• add a regression test covering both representations

Why

The 4.0.16.120061 release failed in actions/setup-java before Gradle started. GitHub's current Temurin catalog exposes the accepted 21.0.7+6 runtime under the selector 21.0.7+6.0.LTS.

Failed run: https://github.com/MinecraftModDevelopmentMods/OreSpawn/actions/runs/33522096942
Companion dispatcher change: #274

Validation

• focused release-workflow contract test passed
• complete clean check build javadoc verifyReleaseArtifacts writeReleaseChecksums gate passed with 134 tests and fresh/reload surface integration
• the main, sources and Javadoc SHA-256 hashes are byte-identical to the accepted candidate
• no production source, gameplay, API, schema or published artifact changed

This PR creates no tag and performs no publication. Both this PR and #274 must merge before retrying the release form.