deps: update googleapis/java-cloud-bom digest to ac9893c (#2472)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| googleapis/java-cloud-bom | action | digest | `8bc17e9` -> `ac9893c` |

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.github/workflows/ci.yaml

@@ -296,7 +296,7 @@ jobs:
       run: |
         mvn install -B -ntp -DskipTests -Dclirr.skip -Dcheckstyle.skip
     - name: Validate gapic-generator-java-bom
-      uses: googleapis/java-cloud-bom/tests/validate-bom@8bc17e9ae3c6354f04df2fdf2d57aeafa63add66
+      uses: googleapis/java-cloud-bom/tests/validate-bom@ac9893c4ba759cda192b11c2e8bac0d5a6bd60a5
       with:
         bom-path: gapic-generator-java-bom/pom.xml