Replace references to master

Author: AkenRoberts

CONTRIBUTING.md

@@ -4,7 +4,8 @@ Thank you for your interest in contributing to Doctrine Extensions!
 
 ## Which Branch Should I Contribute To?
 
-All pull requests should target the `master` branch, with a planned 3.0 major release. 
+All pull requests (new features and bug fixes) should target the `main` branch.
+Anything that can be back-ported to v2.4.x will be done by maintainers.
 
 :warning: The `v.2.4.x` branch has been marked as legacy/deprecated.