ci: change Mergify merge method from merge to rebase (#18)

This commit changes mergify merge method to rebase.

Signed-off-by: Rohan Devasthale <rdevasth@redhat.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rd4398

.mergify.yml

@@ -22,7 +22,7 @@ pull_request_rules:
 
     actions:
       merge:
-        method: merge
+        method: rebase
 
   - name: Ping author on conflicts
     conditions: