chore(mergify): add automatic rebase rule for approved PRs

Add a new Mergify rule to automatically rebase approved PRs when the
base branch (main) is updated. This keeps PRs current with the latest
code and ensures tests run on the actual merge result.

The rebase happens automatically when:
- PR is not a draft
- PR has no merge conflicts
- PR is still open
- PR has at least 1 approval from maintainers

Benefits:
- Prevents stale PRs that fall behind main
- Catches integration issues early
- Ensures tests run on latest codebase
- Maintains cleaner linear commit history

The rebase rule is placed before the auto-merge rule to ensure PRs
are up-to-date before automatic merging occurs.

Signed-off-by: Lalatendu Mohanty <lmohanty@redhat.com>

Author: LalatenduMohanty

.mergify.yml

@@ -1,4 +1,13 @@
 pull_request_rules:
+  - name: Automatic rebase to keep PRs up-to-date
+    conditions:
+      - "-draft"
+      - "-conflict"
+      - "-closed"
+      - "#approved-reviews-by>=1"
+    actions:
+      update:
+        method: rebase
   - name: Add CI label
     conditions:
       - or: