github actions: use korthout/backport-action

.github/workflows/backport.yml

@@ -0,0 +1,35 @@
+name: Backport pull request
+
+on:
+  pull_request_target:
+    types: [closed]
+  issue_comment:
+    types: [created]
+
+permissions:
+  contents: write # so it can comment
+  pull-requests: write # so it can create pull requests
+
+jobs:
+  backport:
+    name: Backport pull request
+    runs-on: ubuntu-24.04
+
+    # Only run when pull request is merged
+    # or when a comment starting with `/backport` is created by someone other than the
+    # https://github.com/backport-action bot user (user id: 97796249). Note that if you use your
+    # own PAT as `github_token`, that you should replace this id with yours.
+    if: >
+        (
+          github.event_name == 'pull_request_target' &&
+          github.event.pull_request.merged
+        ) || (
+          github.event_name == 'issue_comment' &&
+          github.event.issue.pull_request &&
+          github.event.comment.user.id != 97796249 &&
+          startsWith(github.event.comment.body, '/backport')
+        )
+    steps:
+        -   uses: actions/checkout@v4
+        -   name: Create backport pull requests
+            uses: korthout/backport-action@v3