chore: Add mergify[bot] to exception list

https://user-images.githubusercontent.com/13928957/173293263-4987c494-4524-46dd-996c-36f1ca760c68.png
According to this mergify's login id is `mergify[bot]` so am guessing this should work.

.mergify.yml

@@ -7,6 +7,7 @@ pull_request_rules:
           - author!=gavindsouza
           - author!=deepeshgarg007
           - author!=ankush
+          - author!=mergify[bot]
         - or:
           - base=version-13
           - base=version-12