[runners-spark] Spark 4 follow-up: Announce in CHANGES.md and address remaining review comments from #38255 (#38489)

* [runners-spark] Spark 4 follow-up: address remaining review comments from #38255

Addresses two trailing review comments left on PR #38255 at the
approved/merged head SHA that were never folded in before merge:

- Restore the `feature Y added to Java SDK` placeholder line in
  CHANGES.md Highlights (lost to a rebase). Comment:
  #38255 (comment)

- Delete two orphaned `.github/trigger_files/*.json`:
  - `beam_PreCommit_Java_Spark4_Versions.json`: its workflow YAML was
    deleted in #38255.
  - `beam_PostCommit_Java_ValidatesRunner_Spark4StructuredStreaming.json`:
    flagged as "not effective yet"; since then, #38478 renamed the
    associated workflow YAML to
    `beam_PostCommit_Java_ValidatesRunner_Spark4.yml`, so this trigger
    JSON is doubly orphaned.
  Comment:
  #38255 (comment)

The renamed workflow YAML stays in place; per the trigger-files
convention these JSONs are only added when an author wants to manually
trigger a specific PostCommit.

* Update CHANGES.md

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* Fix CHANGES.md URL: /pull/N -> /issues/N for validateChanges

* Update CHANGES.md

Co-authored-by: Yi Hu <huuyyi@gmail.com>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Yi Hu <huuyyi@gmail.com>

Author: 3 people

.github/trigger_files/beam_PostCommit_Java_ValidatesRunner_Spark4StructuredStreaming.json

@@ -60,6 +60,8 @@
 ## Highlights
 
 * New highly anticipated feature X added to Python SDK ([#X](https://github.com/apache/beam/issues/X)).
+* New highly anticipated feature Y added to Java SDK ([#Y](https://github.com/apache/beam/issues/Y)).
+* Spark 4 runner support for Java SDK ([#38255](https://github.com/apache/beam/issues/38255)).
 
 ## I/Os