chore(ci): rework mergify configuration

Author: selfuryon

.github/mergify.yml

@@ -1,12 +1,9 @@
-defaults:
-  actions:
-    queue:
-      allow_merging_configuration_change: true
-
 queue_rules:
   - name: default
     conditions:
       - check-success=buildbot/nix-eval
+    merge:
+      method: squash
 
 pull_request_rules:
   - name: Automerge PRs using the merge queue
@@ -17,9 +14,5 @@ pull_request_rules:
       queue:
         name: default
         method: squash
-
-  - name: Delete head branch after merge
-    conditions:
-      - "merged"
-    actions:
-      delete_head_branch: {}
+      delete_head_branch:
+        force: true