Wagtail 7.2 maintenance

[nickmoreton]

This pull request updates the project's supported Python, Django, and Wagtail versions, dropping older versions and adding support for the latest releases. It also updates development dependencies and adjusts CI and test configurations accordingly.

Platform and framework support changes:

• Dropped support for Python 3.9, Django 5.0, and Wagtail 5.1/5.2; added support for Python 3.13/3.14, Django 5.1/5.2, and Wagtail 7.0/7.1/7.2 in pyproject.toml, tox.ini, README.md

Continuous integration and testing updates:

• Updated GitHub Actions workflow (.github/workflows/test.yml) to test against Python 3.10–3.14 and removed Python 3.9 from the matrix; updated database test combinations to use newer Python versions.
• Refactored tox.ini to align with new Python/Django/Wagtail support.

Development dependencies:

• Upgraded development tools (isort, black, pymarkdown, pyupgrade, django-upgrade) to their latest versions and updated Django target version for code upgrades.

Documentation:

• Updated README.md and CHANGELOG to reflect new platform and framework requirements and supported versions.