Cache Maven dependencies and reuse build output across CI jobs

Every CI lane rebuilds the world, which is where much of the runner time goes, and dependency resolution is repeated per job and per run.

- Cache the Maven repository across jobs and runs. #15749 is a stale earlier attempt at this.
- Share build output between jobs (a build stage plus artifact upload/download) instead of rebuilding per lane.

Part of #19524.

Start by reviewing the repository's CI workflow definitions and the stale earlier attempt in #15749, then read the parent issue #19524 for scope. Done means Maven dependencies are cached across jobs and runs, and build output is uploaded once and reused by each CI lane instead of being rebuilt.

Written by the indexing model from the issue text.