Add Django 5.2 and 6.0 to test matrix, remove Python 3.8 and 3.9

[MarkBird]

Remove unsupported versions of Django and add latest.

[dabapps-robot]

Nice update overall — this keeps the support matrix aligned with current Django/Python versions, and the README/CI are in sync. 👍

A couple of small, non-blocking suggestions:

• Matrix exclusions: the exclude list looks sensible for compatibility, but it might be worth adding a short comment in the workflow explaining why each group is excluded (e.g. Django/Python upstream support window). It’ll help future maintainers when versions move again.
• PR scope: upgrading black/flake8/isort in dev-requirements.txt is reasonable, but it’s a separate concern from runtime test matrix changes. In future, splitting tooling bumps into a separate PR can make reviews and regressions easier to reason about.

Other than that, this looks good to me and is ready to merge.