chore(ci): update backport-assistant to use gh automerge (#14839)

.github/workflows/backport-assistant.yml

@@ -16,11 +16,11 @@ jobs:
   backport:
     if: github.event.pull_request.merged
     runs-on: ubuntu-latest
-    container: hashicorpdev/backport-assistant:0.2.5
+    container: hashicorpdev/backport-assistant:0.3.0
     steps:
       - name: Run Backport Assistant for stable-website
         run: |
-          backport-assistant backport -merge-method=squash -automerge
+          backport-assistant backport -merge-method=squash -gh-automerge
         env:
           BACKPORT_LABEL_REGEXP: "type/docs-(?P<target>cherrypick)"
           BACKPORT_TARGET_TEMPLATE: "stable-website"
@@ -41,13 +41,13 @@ jobs:
           # set BACKPORT_TARGET_TEMPLATE for backport-assistant
           # trims backport/ from the beginning with parameter substitution
           export BACKPORT_TARGET_TEMPLATE="release/${latest_backport_label#backport/}.x"
-          backport-assistant backport -merge-method=squash -automerge
+          backport-assistant backport -merge-method=squash -gh-automerge
         env:
           BACKPORT_LABEL_REGEXP: "type/docs-(?P<target>cherrypick)"
           GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
       - name: Run Backport Assistant for release branches
         run: |
-          backport-assistant backport -merge-method=squash -automerge
+          backport-assistant backport -merge-method=squash -gh-automerge
         env:
           BACKPORT_LABEL_REGEXP: "backport/(?P<target>\\d+\\.\\d+)"
           BACKPORT_TARGET_TEMPLATE: "release/{{.target}}.x"