[skip ci] ci: switch to using automerge

Instead of using merify which seems to not do what we want, use
a combination of mergify and automerge. We let mergify review
dependabot PRs. We let mergify dismiss reviews on updates but
exclude those from Schutzbot. We then let Schutzbot update and
merge the PRs via automerge if the `ci:automerge` label is set.

.github/workflows/automerge.yml

@@ -0,0 +1,31 @@
+name: automerge
+on:
+  pull_request:
+    types:
+      - labeled
+      - unlabeled
+      - synchronize
+      - opened
+      - edited
+      - ready_for_review
+      - reopened
+      - unlocked
+  pull_request_review:
+    types:
+      - submitted
+  check_suite:
+    types:
+      - completed
+  status: {}
+jobs:
+  automerge:
+    runs-on: ubuntu-latest
+    steps:
+      - name: automerge
+        uses: "pascalgn/automerge-action@v0.14.3"
+        env:
+          GITHUB_TOKEN: "${{ secrets.SCHUTZBOT_GITHUB_ACCESS_TOKEN }}"
+          MERGE_LABELS: "ci:automerge"
+          MERGE_REMOVE_LABELS: "ci:automerge"
+          MERGE_METHOD: "rebase"
+          UPDATE_METHOD: "rebase"