Use Smart-mode for "Strict Merge" to enable automatic rebase

Enabling Smart-mode for "Strict Merge" causes Mergify to do a rebase of
the PR before merging. The rebase will trigger CI runs again, and
merging will be done only when the CI results are successful.

See-also: https://docs.mergify.io/actions.html#merge
Signed-off-by: Niels de Vos <ndevos@redhat.com>

.github/mergify.yml

@@ -23,6 +23,8 @@ pull_request_rules:
       merge:
         method: rebase
         rebase_fallback: merge
+        strict: smart
+        strict_method: rebase
       dismiss_reviews: {}
       delete_head_branch: {}
   - name: ask to resolve conflict