Send automated PR upon new stack release

.github/workflows/update.yml

+name: Update Elastic release
+
+on:
+  schedule:
+    - cron: '0 0 * * 0'  # At 00:00 every Sunday
+
+jobs:
+
+  check-and-update:
+    name: Check and update Elastic release
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Get latest release version
+        uses: octokit/request-action@v2.x
+        id: get_latest_release
+        with:
+          route: GET /repos/:repository/releases/latest
+          repository:  elastic/elasticsearch
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Update stack version
+        run: |
+          source .env
+          cur_ver="$ELK_VERSION"
+          new_ver=${{ fromJson(steps.get_latest_release.outputs.data).tag_name }}
+
+          # Escape dot characters so sed interprets them as literal dots
+          cur_ver="$(echo $cur_ver | sed 's/\./\\./g')"
+          # Trim leading 'v' in semantic version
+          new_ver="${new_ver:1}"
+
+          for f in .env docker-stack.yml README.md; do
+            sed -i "s/${cur_ver}/${new_ver}/g" "$f"
+          done
+
+      - name: Send pull request to update to new version
+        uses: peter-evans/create-pull-request@v3
+        with:
+          commit-message: Update to ${{ fromJson(steps.get_latest_release.outputs.data).tag_name }}
+          title: Update to ${{ fromJson(steps.get_latest_release.outputs.data).tag_name }}
+          delete-branch: true