drop mergify (#20770)

* drop mergify
* label conflist

Author: Borda

.github/mergify.yml

@@ -0,0 +1,22 @@
+name: Label conflicts
+
+on:
+  push:
+    branches: ["main"]
+  pull_request_target:
+    types: ["synchronize", "reopened", "opened"]
+
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: false
+
+jobs:
+  triage-conflicts:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: mschilde/auto-label-merge-conflicts@591722e97f3c4142df3eca156ed0dcf2bcd362bd # Oct 25, 2021
+        with:
+          CONFLICT_LABEL_NAME: "has conflicts"
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          MAX_RETRIES: 3
+          WAIT_MS: 5000