Pin treebeard to <4.5

Addresses #6820. Treebeard 4.5 introduces a model change that will generate a phantom _auto migration the next time developers run makemigrations - this will cause migration dependency issues when it's absent on other environments, or on subsequent Wagtail upgrades. We need to coordinate the upgrade to Treebeard 4.5 with a corresponding migration shipped with Wagtail.

setup.py

@@ -23,7 +23,7 @@ install_requires = [
     "Django>=2.2,<3.2",
     "django-modelcluster>=5.1,<6.0",
     "django-taggit>=1.0,<2.0",
-    "django-treebeard>=4.2.0,<5.0",
+    "django-treebeard>=4.2.0,<4.5",
     "djangorestframework>=3.11.1,<4.0",
     "django-filter>=2.2,<3.0",
     "draftjs_exporter>=2.1.5,<3.0",