Prepare 1.4.0 for Wagtail 7.0 through 7.4

[nickmoreton]

Summary

• prepare package metadata for version 1.4.0 with Wagtail 7.0 through 7.4 support on compatible Django 5.2 and 6.0 combinations
• keep Wagtail 7.4 as the locked default contributor stack while testing representative Wagtail 7.0, 7.3, and 7.4 releases
• remove Django 4.2/5.1 and Wagtail 6.x support claims while preserving package behavior
• refresh locked Python dependencies, pre-commit hooks, GitHub Actions, and MySQL CI

Verification

• uv lock --check
• uv run ruff check .
• uv run ruff format . --check
• uv run pre-commit run --all-files
• uv run coverage run manage.py test (17 tests, 100% package coverage)
• all 20 supported SQLite tox environments across Python 3.10-3.14, Django 5.2/6.0, and Wagtail 7.0/7.3/7.4 (17 tests and 100% package coverage each)
• PostgreSQL and MySQL tox environment provisioning
• uv build --sdist --wheel
• uvx twine check dist/*
• built wheel metadata confirms version 1.4.0 and Wagtail>=7.0,<7.5

Hosted SQLite, PostgreSQL, MySQL, and pre-commit checks will run against the pushed update.