Replace Django 6.0a1 with 6.0 in test requirements

Author: vinitkumar

tests/requirements/dj60_cms50.txt

@@ -2,7 +2,7 @@
 
 django-cms>=5.0,<5.1
 
-Django>=6.0a1,<6.1
+Django>=6.0,<6.1
 django-classy-tags
 django-fsm-2==4.1.0
 django-sekizai