Enable CI for sbt 2.0.0-RC2

[xerial]

Summary

• Upgrade sbt version from 2.0.0-M4 to 2.0.0-RC2 for Scala 3 cross-compilation
• Enable CI workflow for sbt 2.x testing
• Add compatibility layer to handle differences between sbt 1.x and 2.0

Changes

• Updated build.sbt to use sbt 2.0.0-RC2 for Scala 3
• Enabled the sbt 2.x CI workflow in .github/workflows/test.yml
• Added JsonFormat instances in PluginCompat for sbt 2.0 compatibility
• Removed Def.derive wrappers that were causing issues in sbt 2.0

Note

Full sbt 2.0 compilation still requires additional work for HashWriter instances and other caching-related changes, but this PR enables CI to start testing sbt 2.0 compatibility.

Test plan

• [x] sbt 1.x compilation works (Scala 2.12)
• [ ] sbt 2.x compilation (partial - CI will test)
• [ ] All existing tests pass

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