Wagtail 7.1 maintenance

[nickmoreton]

This pull request updates the project's test matrix and environment configuration to support multiple versions of Wagtail 7.x, alongside existing Django and Python versions.

Test Matrix Updates:

• Expanded the GitHub Actions test matrix in .github/workflows/test.yml to include jobs for Wagtail 7.1 across all supported Python versions (3.10–3.13), in addition to existing jobs for Wagtail 7.0.

Environment Configuration Updates:

• Modified the tox.ini envlist to explicitly test against both Wagtail 7.0 and 7.1 for each supported Python and Django version, ensuring tests are run for all relevant combinations.
• Updated dependency specifiers in tox.ini to distinguish between wt70 (wagtail>=7.0,<7.1) and wt71 (wagtail>=7.1,<7.2), and fixed a typo in the Django dj52 version specifier.