ci(mergify): upgrade configuration to current format

mergify[bot] · psychon · commit 4f48908bf1e1 · 2026-04-02T16:31:43.000+02:00
diff --git a/.mergify.yml b/.mergify.yml
@@ -6,5 +6,4 @@ pull_request_rules:
     actions:
       merge:
         method: merge
-      delete_head_branch: {}
       dismiss_reviews: {}
