ci: enable automerge for backport PRs (#90)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Expanded pull request auto-merge rules to allow merging into the main
branch and additional release branch patterns, enabling broader
automated merges without changing other conditions or labels.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: nathan-weinberg

.github/mergify.yml

@@ -6,7 +6,9 @@ pull_request_rules:
     - "#approved-reviews-by>=2"
     - "#review-requested=0"
     - "#changes-requested-reviews-by=0"
-    - base=main
+    - or:
+      - base=main
+      - base~=^rhoai-v
     - label!=do-not-merge
     - label!=needs-rebase
     - check-success=pre-commit