Cache Maven distribution and dependencies in CI

[hyperxpro]

Motivation:

Nothing in CI was cached. Every job re-downloaded the wrapper's Maven distribution and the whole local repository on each run, which made builds slow and turned the Maven download into a frequent point of failure.

Modification:

Enable setup-java's Maven cache, which keeps the wrapper distribution under a key that only rotates when the pinned Maven version changes, so dependency bumps no longer evict it. Pull requests read the caches without writing branch-scoped copies.

Add a prefetch-maven composite action that retries the remaining cold fetch, and drop the chmod +x ./mvnw steps since mvnw is already executable in git.

Result:

Maven and its dependencies come from cache, and a transient download failure retries instead of taking the job down.