CI: Add workflow to backport changes

Signed-off-by: Tim Meusel <tim@bastelfreak.de>

Author: bastelfreak

.github/workflows/backport.yml

@@ -0,0 +1,65 @@
+---
+name: Backport merged pull request
+
+
+on:
+  workflow_call: {}
+    secrets:
+      OPENVOXBOT_SSH_PRIVATE_KEY:
+        description: 'ssh private key to sign commits'
+        required: true
+      OPENVOXBOT_COMMIT_AND_PRS:
+        # Provide a fine-grained token with the following repository permissions:
+        # * Contents: Read and write
+        # * Metadata: Read-only (mandatory, default)
+        # * Pull requests: Read and write
+        description: 'PAT to access the GitHub API'
+        required: true
+
+permissions: {}
+
+env:
+  GIT_AUTHOR_NAME: OpenVoxProjectBot
+  GIT_AUTHOR_EMAIL: 215568489+OpenVoxProjectBot@users.noreply.github.com
+  GIT_COMMITTER_NAME: OpenVoxProjectBot
+  GIT_COMMITTER_EMAIL: 215568489+OpenVoxProjectBot@users.noreply.github.com
+  SSH_AUTH_SOCK: /tmp/ssh_agent.sock
+
+jobs:
+  backport:
+    name: Backport merged pull request
+    runs-on: ubuntu-latest
+    # For security reasons, we don't want to checkout and run arbitrary code when
+    # using the pull_request_target trigger. So restrict this to cases where the
+    # backport label is applied to an already merged PR.
+    if: github.event.pull_request.merged && startsWith(github.event.label.name, 'backport ')
+    steps:
+      - name: Add SSH key
+        run: |
+          mkdir -p ~/.ssh
+          echo "${{ secrets.OPENVOXBOT_SSH_PRIVATE_KEY }}" > ~/.ssh/github_actions
+          chmod 600 ~/.ssh/github_actions
+          ssh-agent -a $SSH_AUTH_SOCK > /dev/null
+          ssh-add ~/.ssh/github_actions
+
+      - name: Setup git
+        run: |
+          git config --global user.email "$GIT_AUTHOR_EMAIL"
+          git config --global user.name "$GIT_AUTHOR_NAME"
+          git config --global gpg.format ssh
+          git config --global user.signingkey ~/.ssh/github_actions
+          git config --global commit.gpgsign true
+          git config --global tag.gpgsign true
+      - name: Checkout repository
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          token: ${{ secrets.OPENVOXBOT_COMMIT_AND_PRS }}
+          ref: main
+      - name: Create backport pull requests
+        uses: korthout/backport-action@2e830a1d0b8269505846ddd407a70876913ad1f8 # v4.6.0
+        with:
+          auto_merge_enabled: true
+          auto_merge_method: merge
+          github_token: ${{ secrets.OPENVOXBOT_COMMIT_AND_PRS }}
+          git_committer_name: OpenVoxProjectBot
+          git_committer_email: 215568489+OpenVoxProjectBot@users.noreply.github.com