chore: add branch protection rule (#3247)

In this PR:
- Add branch protection rule for the main branch.

.github/sync-repo-settings.yaml

@@ -1,3 +1,34 @@
 rebaseMergeAllowed: true
 squashMergeAllowed: true
 mergeCommitAllowed: false
+branchProtectionRules:
+- pattern: main
+  isAdminEnforced: true
+  requiredApprovingReviewCount: 1
+  requiresCodeOwnerReviews: true
+  requiresStrictStatusChecks: false
+  requiredStatusCheckContexts:
+  - build(8) except for gapic-generator-java
+  - build(8) for gapic-generator-java
+  - build (11)
+  - build (17)
+  - cla/google
+  - compatibility
+  - graalvm-presubmit-sdk-platform-java-a
+  - graalvm-presubmit-sdk-platform-java-b
+  - library_generation
+  - library-generation-integration-tests
+  - library-generation-lint-python
+  - library-generation-lint-shell
+  - library-generation-unit-tests
+  - lint
+  - should-run-library-generation-tests
+  - showcase (11)
+  - showcase (17)
+permissionRules:
+- team: yoshi-admins
+  permission: admin
+- team: yoshi-java-admins
+  permission: admin
+- team: yoshi-java
+  permission: push