chore: mark the unmanaged dependency check required (googleapis#1845)

b/326247198

Author: suztomo

.github/sync-repo-settings.yaml

@@ -39,7 +39,7 @@ branchProtectionRules:
     - 'Kokoro - Test: Java GraalVM Native Image'
     - 'Kokoro - Test: Java 17 GraalVM Native Image'
     - javadoc
-
+    - unmanaged_dependency_check
 # Identifies the protection rule pattern. Name of the branch to be protected.
 # Defaults to `main`
 - pattern: 1.21.x