chore: mark the unmanaged dependency check required (googleapis#2831)

b/326247198

Author: suztomo

.github/sync-repo-settings.yaml

@@ -18,6 +18,7 @@ branchProtectionRules:
       - 'Kokoro - Test: Java GraalVM Native Image'
       - 'Kokoro - Test: Java 17 GraalVM Native Image'
       - javadoc
+      - unmanaged_dependency_check
   - pattern: 1.113.14-sp
     isAdminEnforced: true
     requiredApprovingReviewCount: 1