Wagtail 7.0 maintenance

[nickmoreton]

This pull request updates the test matrix to include newer versions of Python, Django, and Wagtail, while removing support for older versions. It also introduces concurrency to the GitHub Actions workflow to improve CI efficiency.

Includes: #18

Updates to supported versions:

• Python: Added support for Python 3.13 and removed support for Python 3.8 across the test matrix and classifiers (tox.ini, setup.py).
• Django: Added support for Django 5.1 and 5.2 in the test matrix and classifiers (tox.ini, setup.py).
• Wagtail: Added support for Wagtail 6.2 through 7.0 in the test matrix and classifiers (tox.ini, setup.py).

Continuous Integration improvements:

• GitHub Actions: Introduced a concurrency group to cancel in-progress workflows for the same branch, ensuring only the latest workflow runs.

Documentation updates:

• Changelog: Updated the changelog