Update dependencies and improve build tool testing infrastructure, add JDK 21

[keynmol]

Summary:

• Update dependencies for scip-java itself so it can be built on JDK 21 itself
• Add JDK 21 to build matrix
• Introduce a concept of Tool - which adds the ability to add JDK version ranges, so that the tests are ignored automatically on JDK versions that the tools don't support
• Restore support for Gradle2 and Gradle3 that were quietly lost when a switch to Kotlin init script happened
• Correctly detect external JDK version in build tool tests - previously we'd always detect JDK 11 because that's what the tests were running due to javaHome override

Test plan

• Existing tests that actually run
• New entry in build matrix for JDK 21