Wagtail 7.2 maintenance

[nickmoreton]

This pull request updates the project's supported Python and Wagtail versions + dropping older versions.

Supported Python and Wagtail versions:

• Added support and testing for Python 3.14 and Wagtail 7.2
• Dropped support and testing for Python 3.9 and Wagtail 7.1 across configuration files

Minimum requirements updates:

• Updated minimum required Python version to 3.10

Continuous integration and environment configuration:

• Updated CI workflow matrix to include Python 3.14 and remove Python 3.9

Documentation:

• Updated CHANGELOG.md