requirements/runtime.txt: Django>=2.2,<6.0 (#792)

Fixes #755

We agree to have a full test run before progressing to the next major versions of Django.

* #703 is an example of the kind of crash bugs that can be introduced in Django's major version upgrades.

* #755 describes why we should have confidence in Django minor and micro version upgrades.

Let's work together to add new versions of Django to our GitHub Actions as they are released to provide our users with safe upgrade paths.

requirements/runtime.txt

@@ -1 +1 @@
-Django>=2.2,<5.2
+Django>=2.2,<6.0