Add support for Django 6.0

Django 6.0 has been released so we should add Django 6.0 images.

Important things to note:

• "Following the release of Django 6.0, we suggest that third-party app authors drop support for all versions of Django prior to 5.2." - this suggests we might start deprecating 4.x Django images
• "Django 6.0 supports Python 3.12, 3.13, and 3.14." - this informs the matrix of Python versions we should build.