Instead of always installing and using the latest Gradle version for
cache cleanup, we now require at least Gradle 8.9.
This avoids downloading and installing Gradle if the version on PATH is
sufficient to perform cache cleanup.
The most common case for validation will be that the wrapper jars are unchanged
from a previous workflow run. In this case, we cache the validated wrapper
checksums to minimise the work required on a subsequent run.
Fixes#172
- All cache keys are now structured as 'gradle-<cache-name>-<protocol-version>
- This ensures that extracted entries are prefixed and versioned consistently
- Avoid using custom cache-key prefix for extracted entries. This should reduce the
churn in integration tests that require some level of cache isolation.
