ci: Switch away from mergify to backport action

We want to use github native queues.

.github/workflows/backport.yml

@@ -0,0 +1,37 @@
+name: Backport
+on:
+  pull_request_target:
+    types: [closed, labeled]
+permissions:
+  contents: read
+jobs:
+  backport:
+    name: Backport Pull Request
+    permissions:
+      # for korthout/backport-action
+      contents: write
+      pull-requests: write
+    if: github.repository_owner == 'NixOS' && github.event.pull_request.merged == true && (github.event_name != 'labeled' || startsWith('backport', github.event.label.name))
+    runs-on: ubuntu-24.04-arm
+    steps:
+      - name: Generate GitHub App token
+        id: generate-token
+        uses: actions/create-github-app-token@v1
+        with:
+          app-id: ${{ vars.CI_APP_ID }}
+          private-key: ${{ secrets.CI_APP_PRIVATE_KEY }}
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          # required to find all branches
+          fetch-depth: 0
+      - name: Create backport PRs
+        uses: korthout/backport-action@d07416681cab29bf2661702f925f020aaa962997 # v3.4.1
+        id: backport
+        with:
+          # Config README: https://github.com/korthout/backport-action#backport-action
+          github_token: ${{ steps.generate-token.outputs.token }}
+          github_workspace: ${{ github.workspace }}
+          auto_merge_enabled: true
+          pull_description: |-
+            Automatic backport to `${target_branch}`, triggered by a label in #${pull_number}.