feat: add `backport.yml` (#13163)

* feat: add `backport.yml`

* feat: check releases over labels for latest

* chore: Do not automerge for non `stable-website`

* chore: Do not automerge to not `stable-website`

* Update .github/workflows/backport.yml

Co-authored-by: Meggie <meggie@hashicorp.com>

Co-authored-by: Meggie <meggie@hashicorp.com>

.github/workflows/backport.yml

@@ -0,0 +1,52 @@
+---
+name: Backport Assistant Runner
+
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    if: github.event.pull_request.merged
+    runs-on: ubuntu-latest
+    container: hashicorpdev/backport-assistant:0.2.3
+    steps:
+      - name: Backport changes to stable-website
+        run: |
+          backport-assistant backport -automerge
+        env:
+          BACKPORT_LABEL_REGEXP: "backport/(?P<target>website)"
+          BACKPORT_TARGET_TEMPLATE: "stable-{{.target}}"
+          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
+      - name: Backport changes labeled website to latest release branch
+        run: |
+          resp=$(curl -f -s "https://api.github.com/repos/$GITHUB_REPOSITORY/releases?per_page=100")
+          ret="$?"
+          if [[ "$ret" -ne 0 ]]; then
+              echo "The GitHub API returned $ret"
+              exit $ret
+          fi
+
+          # find the latest non-rc release
+          latest_version=$(echo "$resp" | tr '\r\n' ' ' | jq -r '.[] | select(.name|test("^v\\d+\\.\\d+\\.\\d+$")) | .name' | sort -rV | head -n1)
+          echo "Latest non-rc version: $latest_version"
+
+          # strip leading "v" &
+          target="${latest_version#v}"
+          # replace patch version with "x"
+          target="${target%.*}.x"
+          
+          export BACKPORT_TARGET_TEMPLATE="release/$target"
+          backport-assistant backport
+        env:
+          BACKPORT_LABEL_REGEXP: "backport/(?P<target>website)"
+          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
+      - name: Backport changes to targeted release branch
+        run: |
+          backport-assistant backport
+        env:
+          BACKPORT_LABEL_REGEXP: "backport/(?P<target>\\d+\\.\\d+\\.\\w+)"
+          BACKPORT_TARGET_TEMPLATE: "release/{{.target}}"
+          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}