[jdk25u] RFR: 8370048: Shenandoah: Deprecated ShenandoahPacing option

[William Kemper]

This is not a direct backport. We removed `ShenandoahPacing` from JDK26. This was an experimental option, so we did not formally deprecate it at the time. However, in hindsight, we feel it would be a better user experience in 25 to receive a deprecation warning if the option is given.

Testing:

[0] % ./build/linux-x86_64-server-release/jdk/bin/java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:-ShenandoahPacing -version 
OpenJDK 64-Bit Server VM warning: Option ShenandoahPacing was deprecated in version 25.0 and will likely be removed in a future release.

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Commit messages:
 - Deprecate ShenandoahPacing option in 25 (pacing was removed in 26)

Changes: https://git.openjdk.org/jdk25u/pull/321/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=321&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370048
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk25u/pull/321.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/321/head:pull/321

PR: https://git.openjdk.org/jdk25u/pull/321