ci: javadoc as a required check (#1892)

Manual configuration has been removed. Adding that back via sync-repo-settings.yaml.

Author: suztomo

.github/sync-repo-settings.yaml

@@ -18,6 +18,7 @@ branchProtectionRules:
       - OwlBot Post Processor
       - 'Kokoro - Test: Java GraalVM Native Image'
       - 'Kokoro - Test: Java 17 GraalVM Native Image'
+      - javadoc
   - pattern: 1.22.0-sp
     isAdminEnforced: true
     requiredApprovingReviewCount: 1