fix JVM toolchain download

[jonathanlermitage]

The build.gradle.kts defines a JVM toolchain, but if no compatible JDK is found, no JDK is downloaded and the build fails.
To download a JDK automatically if needed, we need the org.gradle.toolchains.foojay-resolver-convention plugin. Tested on Arch Linux, with JDK21 installed: after adding the foojay plugin, ./gradlew buildPlugin no longer fails, and it downloads a compatible JDK17.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.92%. Comparing base (ae35708) to head (dbe7b10).
Report is 85 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #473      +/-   ##
==========================================
- Coverage   27.58%   26.92%   -0.67%     
==========================================
  Files           4        4              
  Lines          29       26       -3     
==========================================
- Hits            8        7       -1     
+ Misses         21       19       -2     

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