.mergify: Fix pull_request_rules deprecation

The following message is visible on mergify runs:

"The configuration uses the deprecated merge_method attribute of the
queue action in one or more pull_request_rules. It must now be used
under the queue_rules configuration."

With the following warning:

"A brownout is planned on August 26th, 2024.
This option will be removed on September 23rd, 2024."

This change updates the configuration file to comply with the new
format.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

.mergify/config.yml

@@ -26,9 +26,10 @@
 
 queue_rules:
   - name: default
-    conditions:
+    queue_conditions:
       - base~=(^main|^master|^stable/)
       - label=push
+    merge_method: rebase
 
 pull_request_rules:
   - name: Automatically merge a PR when all required checks pass and 'push' label is present
@@ -37,7 +38,6 @@ pull_request_rules:
       - label=push
     actions:
       queue:
-        method: rebase
         name: default
 
   - name: Post a comment on a PR that can not be merged due to a merge conflict