ci(mergify): upgrade configuration to current format

Author: mergify[bot]

.github/mergify.yml

@@ -6,9 +6,9 @@ pull_request_rules:
           - author=trace4cats-steward[bot]
     actions:
       request_reviews:
-        users: [ janstenpickle, catostrophe ]
+        users: [janstenpickle, catostrophe]
       label:
-        add: [ dependency-update ]
+        add: [dependency-update]
 
   - name: Merge Stewards' PRs
     conditions:
@@ -35,8 +35,3 @@ pull_request_rules:
       merge:
         method: rebase
 
-  - name: Delete head branch after merge
-    conditions:
-      - merged
-    actions:
-      delete_head_branch: