Relax Django upper bound in install_requires to the next major version (6.0)

This allows the more "adventurous" to update their requirements so they
can install the next latest Django version (5.1 and 5.2) even if we
haven't officially supported it. As long as we're on top of Django's
updates, this should be OK for the duration of this Wagtail release's
active support period.

However, do not update the default project template's requirements yet,
since we don't want new projects to be installed with Django versions we
haven't officially supported.

setup.py

@@ -19,7 +19,7 @@ except ImportError:
 
 
 install_requires = [
-    "Django>=4.2,<5.1",
+    "Django>=4.2,<6.0",
     "django-modelcluster>=6.1,<7.0",
     "django-permissionedforms>=0.1,<1.0",
     "django-taggit>=2.0,<5.0",