Use main for reusable WF in backport-issue.yml

**Auto-cherry-pick to `6.5.x` & `6.4.x`**

Author: artembilan

.github/workflows/backport-issue.yml

@@ -7,6 +7,6 @@ on:
 
 jobs:
   backport-issue:
-    uses: spring-io/spring-github-workflows/.github/workflows/spring-backport-issue.yml@v5
+    uses: spring-io/spring-github-workflows/.github/workflows/spring-backport-issue.yml@main
     secrets:
       GH_ACTIONS_REPO_TOKEN: ${{ secrets.GH_ACTIONS_REPO_TOKEN }}