With improved cache-cleanup configurability in Gradle 8 and further improvements in Gradle 8.8, we should simplify the code used to force the cleanup of all unused entries.

This will tie the cache-cleanup implementation to Gradle 8.8+, and we'll need to ensure that Gradle 8.8 is always used when running the cache-cleanup action. Provisioning the Gradle version to use for cache-cleanup will also address #33.