Notes:
- Gradle 9+ requires Java 17 to run
- using Java 21 on GitHub Actions
- not using Java 25 because Kotlin does not yet support it and output some warnings
- default is now Java 8 toolchain (to fix Eclipse project import)
- `src/main/module-info` and `src/main/java9` are compiled with Java 11 toolchain (if global toolchain is Java 8)
- `src/main/module-info` and `src/main/java9` are no longer imported into Eclipse projects
- task `errorprone` now uses at least Java 11 toolchain