[skip ci] ci: preserve reviews across rebase

Re-review the PR after rebasing it. Leave a message to make it
clear that it was not the impersonated person but mergify that
did it.
Ideally, if we had premium, we would impersonate Schutzbot so
it is clear who did it and then use mergify to dismiss reviews
on changes but not for Schutzbot.

.github/mergify.yml

@@ -67,6 +67,9 @@ pull_request_rules:
         update_method: rebase
         rebase_fallback: none
         require_branch_protection: false
+      review:
+        type: APPROVE
+        message: Automatic review via mergify
   - name: Remove label after merge or close
     conditions:
       - merged