fix: use setup-gradle for Gradle caching in CI

[d4rken]

Summary

Every Run tests CI job emits:

[warning]Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.

Root cause: the run-tests job in code-checks.yml runs inside container: ubuntu:24.04. The actions/cache steps in .github/actions/common-setup/action.yml target ~/.gradle/wrapper and ~/.gradle/caches, which resolve to a container-local home dir that does not match where Gradle actually writes its caches in CI. actions/cache validates the paths at save time, finds nothing, warns, and skips the save. The Gradle cache is therefore never persisted between runs (the audit estimates ~8min added to :check each time).

Fix

Replace the two manual actions/cache steps with a single gradle/actions/setup-gradle@v6.1.0 step. This action:

• Auto-detects GRADLE_USER_HOME correctly on both host runners and container jobs.
• Uses Gradle-aware cache keys (no manual hashFiles glob to maintain).
• Is the Gradle-team-recommended approach and supersedes the older manual actions/cache pattern.

Pinned to 50e97c2cd7a37755bbfafc9c5b7cafaece252f6e (#v6.1.0), the same SHA already used by gradle-wrapper-validation.yml.

Follow-up (not in this PR)

The build also surfaces:

Consider enabling configuration cache to speed up this build

That requires Gradle code changes to verify all tasks are config-cache compatible, so it's out of scope here. Tracked as a separate follow-up.

Test plan

• CI runs without the Path Validation Error warning
• Second CI run on this branch reports a cache hit (gradle/actions/setup-gradle logs Restored Gradle User Home from cache)
• All existing jobs (build-modules, build-docker-image, run-tests, regression-synthetic-replay) still pass