ci(mergify): upgrade configuration to current format

.github/mergify.yml

@@ -144,13 +144,13 @@ pull_request_rules:
       - "head~=(?i)^(bug)?fix" # if the PR branch starts with (bug)?fix/
     actions:
       label:
-        add: [ "bug" ]
+        add: ["bug"]
   - name: Documentation label
     conditions:
       - "head~=(?i)^docs" # if the PR branch starts with docs/
     actions:
       label:
-        add: [ "documentation" ]
+        add: ["documentation"]
   - name: Feature label
     conditions:
       - "head~=(?i)^feat(ure)?" # if the PR branch starts with feat(ure)?/
@@ -202,16 +202,7 @@ pull_request_rules:
         message: |
           This pull request looks stale. Feel free to reopen it if you think it's a mistake.
       label:
-        add: [ "stale" ]
-  # ===============================================================================
-  # BRANCHES
-  # ===============================================================================
-
-  - name: Delete head branch after merge
-    conditions:
-      - merged
-    actions:
-      delete_head_branch:
+        add: ["stale"]
 
   #- name: automatic update for PR marked as “Ready-to-Go“
   #  conditions:
@@ -241,4 +232,4 @@ pull_request_rules:
   #      summary: |
   #        {% if not check_succeed %}
   #        Your pull request title must follow [Conventional Commit](https://www.conventionalcommits.org/en/v1.0.0/).
-  #        {% endif %}
+  #        {% endif %}