- 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
- always use Java toolchains
- default is Java 11 (but source/targetCompatibility is still 1.8)
- use system property `toolchain` (e.g. `-Dtoolchain=25`) to compile with other Java versions
preparation for Gradle 9.x, which requires Java 17+ to run