Wagtail 71 maintenance

[nickmoreton]

This pull request updates the project's compatibility and testing matrix to support newer versions of Django and Wagtail.

Dependency and Testing Matrix Updates:

• Updated tox.ini to add testing environments for Django 5.2 and Wagtail 7.0/7.1, and removed environments for Django 5.0, Wagtail 5.2, and Wagtail 6.4.
• Updated dependency requirements in pyproject.toml to require Wagtail >=6.3 and added classifiers for Django 5.2 and Wagtail 7, while removing Django 5.0 and Wagtail 5 classifiers.
• Updated the changelog in CHANGELOG.md to reflect the new testing matrix and dropped support for older versions.
• Updated dependency installation and environment definitions in tox.ini for Django 5.2 and Wagtail 7.0/7.1.

Test Runner Improvements:

• Modified runtests.py to ensure the tests directory is in sys.path for proper Django imports.