Wagtail 6.4 compatibility

[ianmeigh]

This pull request introduces changes to support Wagtail 6.4.

• .github/workflows/test.yml:
  • Updated the latest test to use Wagtail 6.4 and the legacy test to use Wagtail 6.3 (LTS) in addition to version 5.2.
• tox.ini:
  • Added Wagtail 6.4 to the testing environments and removed out-of-support versions of Wagtail (6.1 and 6.2).
• tests/test_image_chooser_views.py:
  • Added the new default_alt_text image property to test assertions
• pyproject.toml:
  • Added Python 3.13 trove classifier
• README.md:
  • Removed removed out-of-support versions of Wagtail (6.0-6.2).
• CHANGELOG.md:
  • Added draft unreleased section

[zerolab]

Closing in favour of #39