actions: backport: Update action and run only if the org PR is merged

This is taken directly from:

zephyrproject-rtos/zephyr@260d054

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

.github/workflows/backport.yml

@@ -4,13 +4,28 @@ on:
     types:
       - closed
       - labeled
+    branches:
+      - main
 
 jobs:
   backport:
-    runs-on: ubuntu-20.04
     name: Backport
+    runs-on: ubuntu-22.04
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged &&
+      (
+        github.event.action == 'closed' ||
+        (
+          github.event.action == 'labeled' &&
+          contains(github.event.label.name, 'backport')
+        )
+      )
     steps:
       - name: Backport
-        uses: zephyrproject-rtos/action-backport@v1.1.99
+        uses: zephyrproject-rtos/action-backport@v2.0.3-3
         with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
+          github_token: ${{ secrets.NCS_GITHUB_TOKEN }}
+          issue_labels: Backport
+          labels_template: '["Backport"]'