Wagtail 7.0 & 7.1 maintenance

[nickmoreton]

This pull request updates the package to support and test against newer versions of Wagtail and Django, and drops support for some older versions.

Framework and Dependency Updates:

• Updated dependencies to require Wagtail>=6.3 and Django>=4.2, and increased minimum pyairtable and djangorestframework versions. Minimum supported Python version is now 3.9.
• Updated PyPI classifiers to reflect support for Django 4.2, 5.1, 5.2 and Wagtail 7, and removed classifiers for Django 5.0.

Testing Matrix and Environment Changes:

• Updated tox.ini to test against Wagtail 6.3, 7.0, and 7.1, and Django 4.2, 5.1, and 5.2, removing older versions from the test matrix.
• Adjusted tox.ini dependency groups to match the new combinations of Django and Wagtail versions, and removed references to dropped versions.

Documentation:

• Added an Unreleased section to CHANGELOG.md noting Wagtail 7.0 & 7.1 maintenance.