ci: Clean up old release PR before creating a new one

.github/workflows/release-plz.yml

@@ -21,6 +21,14 @@ jobs:
           toolchain: nightly
           override: true
           profile: minimal
+      - name: Close old release PR
+        run: |
+          # List all opened PRs which head branch starts with "release-plz-"
+          release_pr=$(gh pr list --state='open' --json number,headRefName --jq '.[] | select(.headRefName | startswith("release-plz-")) | .number')
+          # Close the release PR if there is one
+          if [[ -n "release_pr" ]]; then
+            gh pr close $release_pr
+          fi
       - name: Run release-plz
         uses: MarcoIeni/release-plz-action@v0.5
         env: