Use backport-bot GHA directly

The `spring-io/spring-github-workflows/.github/workflows/spring-backport-issue`
now just delegates directly to the `spring-io/backport-bot`.
Therefore, there is no reason in extra reusable workflow layer on the matter.

* Fix `backport-issue.yml` to use `spring-io/backport-bot` directly

**Auto-cherry-pick to `5.1.x` & `5.0.x`**

Author: artembilan

.github/workflows/backport-issue.yml

@@ -7,6 +7,9 @@ on:
 
 jobs:
   backport-issue:
-    uses: spring-io/spring-github-workflows/.github/workflows/spring-backport-issue.yml@v7
-    secrets:
-      GH_ACTIONS_REPO_TOKEN: ${{ secrets.GH_ACTIONS_REPO_TOKEN }}
+    if: contains(github.event.head_commit.message, 'Fixes:') || contains(github.event.head_commit.message, 'Closes:')
+    runs-on: ubuntu-latest
+    steps:
+      - uses: spring-io/backport-bot@v0.0.1
+        with:
+          token: ${{ secrets.GH_ACTIONS_REPO_TOKEN }}