Reach the ASF SHOULD target of at most 15 concurrent CI runners

After the trim in #19514, a push peaks at around 16 concurrent runners. The [ASF policy](https://infra.apache.org/github-actions-policy.html) says concurrency SHOULD be at most 15.

`max-parallel` does not help: it caps matrix entries within a job, and every surviving job has exactly one entry, so peak concurrency equals the job count. Merging `test-flink-1` with `test-flink-2`, and the two bundle-validation jobs, gets there and is free in wall clock, since both merged jobs (69 min and 25 min measured) stay inside the critical path set by `test-spark-java17-java-tests-part1` at roughly 70 min.

Part of #19524.

Locate the GitHub Actions workflow containing test-flink-1, test-flink-2, and the two bundle-validation jobs; inspect how those jobs are defined and how their dependencies affect concurrency. Merge the specified job pairs, then verify that peak concurrency is at most 15 and the merged jobs remain within the existing critical path.

Written by the indexing model from the issue text.