Wagtail 7.2 maintenance

[nickmoreton]

This pull request updates the project's testing matrix and configuration files to add support for Python 3.14 and Wagtail 7.2

Testing and environment updates:

• Added Python 3.14 as a supported version in setup.cfg classifiers and tox.ini environment definitions.
• Updated tox.ini to include Wagtail 7.2 in the test matrix and removed references to Wagtail 7.1
• Added a test environment for Python 3.14, Django 5.2, and Wagtail 7.2 in tox.ini.
• Documented the addition of Python 3.14 and Wagtail 7.2 testing in CHANGELOG.rst.