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 is a sensible refresh of the support matrix (dropping EOL Python/Django versions and adding current ones), and the README docs link update is a good touch.

A couple of things I’d like to double-check before merge:

1. GitHub Action major versions

   • You’ve bumped to:
     • actions/checkout@v6
     • actions/setup-python@v6
     • actions/cache@v5
   • Please confirm these tags exist and are intended. I’d expect current majors to be lower (checkout/cache/setup-python versions tend to lag those numbers), so this could break workflow resolution if they’re not published tags.

2. Python 3.14 in matrix

   • If 3.14 is still pre-release in your target environment, you may need allow-prereleases: true for setup-python, or hold 3.14 until stable availability.
   • If it’s already stable for your CI environment, then this is fine.

If those two points are confirmed/adjusted, this looks good to merge.