CI: support Ubuntu 26.04 and restore ubuntu-latest runners #2246

Context

PR #2245 temporarily pins CI runners to Ubuntu 24.04 while fixing the Spark lane's JVM. PR #2244 also changes CI runners to Ubuntu 24.04. The review comment asks that we track Ubuntu 26 support.

On the Ubuntu 26.04 image selected by ubuntu-latest, actions/setup-python@v5 failed before tests because Python 3.8 and 3.9 builds were unavailable (e.g. #2245 run 40219220790, py3.9 lint and py3.8 NetworkX lanes). The Spark lane also needs an explicit supported JDK; #2245 pins Temurin 17. The Ubuntu 24.04 changes are a temporary CI compatibility measure, not the desired long-term runner policy.

Acceptance criteria

• Establish a supported way to run every still-supported Python version, including 3.8/3.9 unless the project separately approves a support-floor change, on Ubuntu 26.04. Keep the current test matrix and coverage gates meaningful; no silent skips.
• Verify the Spark lane on Ubuntu 26.04 with the explicit supported JDK, and resolve any other runner-image incompatibilities revealed by full CI.
• On an exact PR head, obtain terminal green CI, CodeQL, workflow security, TCK, route-off, and docs/RTD gates applicable to the workflow change; retain logs showing the legacy Python and Spark lanes actually executed.
• Restore the affected CI jobs to ubuntu-latest by removing the temporary Ubuntu 24.04 pins introduced across Fix remote Let/DAG identity fallback #2244/ci(spark): pin Temurin 17 for PySpark tests #2245, then verify the resulting master workflow. Avoid a release dependency unless the support policy itself changes.

Track each remaining failure by job/root cause; do not treat a skipped lane or Python support-floor reduction as Ubuntu 26 compatibility proof.