CI: Fix automerge action

- Add version, which is required by Github.
- Increase number of retries and sleep time between them to cover how
long it takes to run our test suite.
- Make the workflow fail on error.

.github/workflows/automerge.yml

@@ -25,10 +25,13 @@ jobs:
     steps:
       - id: automerge
         name: Automerge
-        uses: "pascalgn/automerge-action"
+        uses: "pascalgn/automerge-action@v0.16.4"
         env:
           GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
           MERGE_LABELS: "automerge"
           MERGE_REMOVE_LABELS: "automerge"
           MERGE_METHOD: "squash"
           BASE_BRANCHES: "6.x"
+          MERGE_RETRIES: "12"
+          MERGE_RETRY_SLEEP: "300000"
+          MERGE_ERROR_FAIL: "true"