Update beam_PreCommit_Whitespace.yml to run validateChanges (apache#36064)

* Update beam_PreCommit_Whitespace.yml to run validateChanges

fixes apache#35760

* Update beam_PreCommit_Whitespace.yml

* Update beam_PreCommit_Whitespace.yml

Author: liferoad

.github/workflows/beam_PreCommit_Whitespace.yml

@@ -86,3 +86,8 @@ jobs:
         uses: ./.github/actions/gradle-command-self-hosted-action
         with:
           gradle-command: :whitespacePreCommit
+      - name: validate CHANGES.md
+        uses: ./.github/actions/gradle-command-self-hosted-action
+        with:
+          gradle-command: :validateChanges
+