CI: Select Flink PR matrix by changed version

[ajantha-bhat]

depends on #16945

Summary

This updates Flink CI to build an incremental PR matrix based on the changed Flink version path.

For ordinary PRs, Flink CI now uses the JVM plan from the earlier Java CI PR and then narrows the Flink matrix when changes are scoped to a known Flink version path:

• flink/v1.20/** runs Flink 1.20 only
• flink/v2.0/** runs Flink 2.0 only
• flink/v2.1/** runs Flink 2.1 only
• unknown Flink version paths, unknown Flink paths, missing changed-file detection, full-ci, and push/main/release/tag runs fall back to the full Flink matrix

Known Flink versions are read from knownFlinkVersions in gradle.properties, so adding/removing Flink versions does not require hard-coding them in the planner.

This keeps the PR path conservative: stale or unknown planner inputs run more CI, not less.

Testing

• actionlint .github/workflows/flink-ci.yml
• bash -n .github/scripts/ci-plan-common.sh .github/scripts/plan-flink-ci.sh
• YAML parse for jvm-ci-plan.yml and flink-ci.yml
• git diff --check
• Local planner checks:
  • flink/v2.0/** -> Flink 2.0 only
  • flink/v1.20/** -> Flink 1.20 only
  • unknown Flink version/path -> all Flink versions
  • full-ci / push -> all Flink versions with the full JVM matrix