Bump Gradle to 8

[artamonovkirill]

• Gradle 8 supports Java up to 23: https://docs.gradle.org/current/userguide/compatibility.html#java_runtime
• A little fix for list-ops reference solution to work in all JDKs
• Verify on CI that Gradle files were updated

[github-actions]

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[artamonovkirill]

CI is expected to fall on "Validation" job, I haven't run ./gradlew wrapper task to not mix my and auto-generated changes.

[glennj]

LGTM. auto-generate away!

[artamonovkirill]

CI is expected to fall on "Validation" job, I haven't run ./gradlew wrapper task not to mix my and auto-generated changes.

I was wrong, wrapper-validation GitHub actions only verifies gradle-wrapper.jar fires are not corrupted (by comparing checksums)

[artamonovkirill]

We can add below two steps to the validation job to ensure the ./gradlew wrapper command was actually run:

      - uses: eskatos/gradle-command-action@ac2d340dc04d9e1113182899e983b5400c17cda1
        with:
          arguments: wrapper
      - run: git status --porcelain