ci: always add target:release label when backporting

Previously the label was not added if the backport PR was created
manually. The new code is also easier to maintain as it's close to other
label-related code.

(cherry picked from commit 4c788b1757)

.github/workflows/backport.yml

@@ -26,14 +26,3 @@ jobs:
           pull_title: "${pull_title}"
           label_pattern: "^ci:backport ([^ ]+)$"
           github_token: ${{ steps.app-token.outputs.token }}
-
-      - if: ${{steps.backport.outputs.was_successful == 'true'}}
-        uses: actions/github-script@v7
-        with:
-          script: |
-            github.rest.issues.addLabels({
-              issue_number: ${{steps.backport.outputs.created_pull_numbers}},
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              labels: ['target:release']
-            })

.github/workflows/labeler_pr.yml

@@ -33,8 +33,25 @@ jobs:
     - name: "Extract if the PR is a breaking change and add it as label"
       run: gh pr edit "$PR_NUMBER" --add-label "$(echo "$PR_TITLE" | sed -E 's|[[:alpha:]]+(\(.*\))?!:.*|breaking-change|')" || true
 
-  request-reviewer:
+  target-release:
     needs: ["changed-files", "type-scope"]
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+    - if: startsWith(github.base_ref, 'release')
+      uses: actions/github-script@v7
+      with:
+        script: |
+          github.rest.issues.addLabels({
+            issue_number: context.issue.number,
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            labels: ['target:release']
+          })
+
+  request-reviewer:
+    needs: ["changed-files", "type-scope", "target-release"]
     permissions:
       pull-requests: write
     uses: ./.github/workflows/reviewers_add.yml