CI: Select Flink PR matrix by changed version

[ajantha-bhat]

Summary

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

For pull requests, Flink CI runs on JDK 17 and 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, and missing changed-file detection fall back to the full Flink version matrix

For non-PR runs, including main branch, release branch, and tag builds, Flink CI still runs the full Flink version matrix with both JDK 17 and JDK 21.

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

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

Testing

• zizmor --min-severity medium --min-confidence medium --no-progress .github/workflows/flink-ci.yml
• actionlint .github/workflows/flink-ci.yml
• bash -n .github/scripts/ci-plan-common.sh .github/scripts/plan-flink-ci.sh
• YAML parse for flink-ci.yml
• git diff --check origin/main..HEAD
• Local planner checks:
  • flink/v2.0/** -> Flink 2.0 only on JDK 17 for PRs
  • unknown Flink version/path -> all known Flink versions on JDK 17 for PRs
  • non-PR run -> all known Flink versions with JDK 17 and 21

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AI Disclosure

• Model: GPT-5 Codex
• Platform/Tool: Codex
• Human Oversight: partially reviewed
• Prompt Summary: Update Flink CI to select the PR matrix incrementally by changed Flink version path.