Support/wagtail 7.0 - #38

This PR updates the test matrix for Wagtail 7 on top of the changes added in #37

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