chore: remove outdated announcements

Author: Sam Harrison

README.md

@@ -11,8 +11,6 @@
 If you are looking for the SendGrid API client library, please see [this repo](https://github.com/sendgrid/sendgrid-php).
 
 # Announcements
-**The default branch name for this repository has been changed to `main` as of 07/27/2020.**
-
 All updates to this library are documented in our [CHANGELOG](CHANGELOG.md).
 
 # Table of Contents