Updated Git branch "master" to "main".
Updated Git branch "master" to "main".

This change follows a long discussion on django-develops:

https://groups.google.com/g/django-developers/c/tctDuKUGosc/

Refs django/django#14048
django/code.djangoproject.com,django/code.djangoproject.com,django/code.djangoproject.com,django/code.djangoproject.com