[improve][build] Switch default JDK to 25 for 5.0.0-M2

[lhotari]

Motivation

Making JDK 25 the default was agreed on the dev@ mailing list:
https://lists.apache.org/thread/3zclb8cq0dt95c8f1lh83qxyyj11m8pq

As part of the release planning, 4.3.0 was renamed to 5.0.0-M1:
https://lists.apache.org/thread/1hyrcq11jbczdkmqqw07h5qq6fyfxoj7

5.0.0-M1 ships with Java 21 as the default and is being released from the
already-cut branch-5.0-M1. The switch to Java 25 lands in 5.0.0-M2, which
is the master development line now that the M1 release branch exists. This change
therefore targets master only; branch-5.0-M1 keeps Java 21 as its default.

Modifications

• gradle/libs.versions.toml: docker-jdk 21 → 25, so the official Pulsar
  Docker image is built on JDK 25 by default.
• .github/workflows/pulsar-ci.yaml and .github/workflows/pulsar-ci-flaky.yaml
  — make JDK 25 the primary JDK and keep JDK 21 as the secondary scheduled run (swap):
  • workflow_dispatch jdk_major_version input: default 21 → 25 (options reordered 25, 21).
  • Select JDK major version step: the default fallback (pull requests, pushes, and the
    0 12 * * * scheduled run) is now JDK 25; the special-cased 0 6 * * * scheduled run
    now uses JDK 21.
  • Updated the schedule comments to match the new mapping.
• Left unchanged on purpose: the runtime_jdk: 17 / 21 / 25 shade-run and Native Image
  matrix entries — those exercise specific runtime JDKs regardless of the primary build JDK,
  so they are not part of the swap.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial build/CI configuration change without dedicated test coverage; it is
exercised by the CI workflows themselves, which now build and test on JDK 25 by default.

Does this pull request potentially affect one of the following parts:

• Anything that affects deployment

The official Pulsar Docker image is now built on JDK 25 by default (docker-jdk = "25").
This applies to the master / 5.0.0-M2 line only; the 5.0.0-M1 release (branch-5.0-M1)
continues to default to Java 21.