feat: setting up 2.15.x (#1880)

* feat: setting up 2.15.x

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: sydney-munro

.github/release-please.yml

@@ -26,3 +26,7 @@ branches:
     handleGHRelease: true
     releaseType: java-backport
     branch: 2.6.x
+  - bumpMinorPreMajor: true
+    handleGHRelease: true
+    releaseType: java-backport
+    branch: 2.15.x

.github/sync-repo-settings.yaml

@@ -80,6 +80,23 @@ branchProtectionRules:
       - OwlBot Post Processor
       - 'Kokoro - Test: Java GraalVM Native Image'
       - 'Kokoro - Test: Java 17 GraalVM Native Image'
+  - pattern: 2.15.x
+    isAdminEnforced: true
+    requiredApprovingReviewCount: 1
+    requiresCodeOwnerReviews: true
+    requiresStrictStatusChecks: false
+    requiredStatusCheckContexts:
+      - dependencies (8)
+      - dependencies (11)
+      - lint
+      - clirr
+      - units (8)
+      - units (11)
+      - 'Kokoro - Test: Integration'
+      - cla/google
+      - OwlBot Post Processor
+      - 'Kokoro - Test: Java GraalVM Native Image'
+      - 'Kokoro - Test: Java 17 GraalVM Native Image'
 permissionRules:
   - team: yoshi-admins
     permission: admin