Update 'all tests' GitHub action, enable Gradle Build Cache

[aSemy]

This PR contains some proposals for improving the 'all tests' GitHub action. I hope to make them faster.

I'm making a PR before an issue to test/demo that it works as intended :)

• Only test LTS JDK versions (11, 17), plus the latest version (18 at the moment - this can change more frequently). I'm making an assumption here that there's not going to be major differences between the non-LTS versions, and even if there are, as far as I know they're not well supported.

  Reducing the number of versions improves the actions speed, as I think there's a limit on how many jobs can run in parallel.

• Add 'concurrency'. I've noticed when I've pushed a few changes in rapid succession, each change will be tested. This isn't necessary - the tests should be cancelled and restarted if there's a newer commit.

  I copied the concurrency config from here: https://docs.github.com/en/actions/examples/using-concurrency-expressions-and-a-test-matrix#understanding-the-example

• I bumped Kotlin 1.6.0 to 1.6.21. It seemed odd that 1.5 and 1.7 were on the latest version, but 1.6 wasn't.

• I updated the Gradle caching, copied from

  https://github.com/JetBrains/intellij-platform-plugin-template/actions/runs/242898088/workflow#L46-L58

  This means the cache key is less specific (I think sharing the Gradle cache between Kotlin & JVM versions is fine, Gradle will rebuild/reuse tasks if necessary), and the wrapper and dependencies are cached separately (if the dependencies change, the wrapper can still be cached)

• I enabled Gradle Build cache. This should really help build speeds.

[Raibaz]

This is awesome, thanks so much!