ci: javadoc as a required check (#345)

Manual configuration of the required check at Settings tab has been reverted. Adding that back via sync-repo-settings.yaml.

Author: suztomo

.github/sync-repo-settings.yaml

@@ -16,6 +16,7 @@ branchProtectionRules:
       - 'Kokoro - Test: Integration'
       - cla/google
       - OwlBot Post Processor
+      - javadoc
   - pattern: java7
     isAdminEnforced: true
     requiredApprovingReviewCount: 1