chore: change default branches to main (#5107)

Author: bshaffer

CONTRIBUTING.md

@@ -6,5 +6,5 @@ and review pull requests against the main
 repository, which contains all of our client libraries. You will also need to
 sign a Contributor License Agreement. For more details about how to contribute,
 see the
-[CONTRIBUTING.md](https://github.com/googleapis/google-cloud-php/blob/master/CONTRIBUTING.md)
+[CONTRIBUTING.md](https://github.com/googleapis/google-cloud-php/blob/main/CONTRIBUTING.md)
 file in the main Google Cloud PHP repository.

README.md

@@ -31,7 +31,7 @@ please see our [gRPC installation guide](https://cloud.google.com/php/grpc).
 
 ### Authentication
 
-Please see our [Authentication guide](https://github.com/googleapis/google-cloud-php/blob/master/AUTHENTICATION.md) for more information
+Please see our [Authentication guide](https://github.com/googleapis/google-cloud-php/blob/main/AUTHENTICATION.md) for more information
 on authenticating your client. Once authenticated, you'll be ready to start making requests.
 
 ### Sample