Update references to release branches

Prepare for renaming `v1` -> `releases/v1` and `v2` -> `releases/v2`.

.github/workflows/post-release-mergeback.yml

@@ -15,8 +15,8 @@ on:
 
   push:
     branches:
-      - v1
-      - v2
+      - releases/v1
+      - releases/v2
 
 jobs:
   merge-back:
@@ -106,7 +106,7 @@ jobs:
           git push origin --atomic --force refs/tags/"${VERSION}" refs/tags/"${major_version_tag}"
 
       - name: Create mergeback branch
-        if: steps.check.outputs.exists != 'true' && contains(github.ref, 'v2')
+        if: steps.check.outputs.exists != 'true' && contains(github.ref, 'releases/v2')
         env:
           VERSION: "${{ steps.getVersion.outputs.version }}"
           NEW_BRANCH: "${{ steps.getVersion.outputs.newBranch }}"