ci(Mergify): configuration update

Signed-off-by: James Cole <james@firefly-iii.org>

.github/mergify.yml

@@ -1,5 +1,11 @@
----
 pull_request_rules:
+  - name: Make sure PR are up to date before merging
+    description: This automatically updates PRs when they are out-of-date with the
+      base branch to avoid semantic conflicts (next step is using a merge
+      queue).
+    conditions: []
+    actions:
+      update:
   - name: Close all on main
     conditions:
       - base=main