Wagtail 74 maintenence

[nickmoreton]

Summary

Maintenance update adding Wagtail 7.4 LTS support and rounding out the Django 5.2 / Python 3.14 test matrix.

• Add Wagtail 7.4 LTS to the tox matrix for both Django 5.2 and Django 6.0 environments
• Add Python 3.14 to the Django 5.2 environments (supported since Django 5.2.8)
• Fix wagtailcaptcha/__init__.py version (2.1.1 → 2.3.0)
• Update README compatibility table and CHANGELOG for 2.3.0
• Bump dev tooling: ruff v0.15.12, pyproject-fmt v2.21.2, coverage minimum >=7

Supported versions after this PR

Axis	Versions
Python	3.10, 3.11, 3.12, 3.13, 3.14
Django	5.2 LTS, 6.0
Wagtail	7.0 LTS, 7.3, 7.4 LTS

Wagtail 7.1 and 7.2 are intentionally omitted as they are EOL feature releases.

Test plan

• CI matrix passes across Python 3.10–3.14 with Django 5.2 / 6.0 and Wagtail 7.0 / 7.3 / 7.4
• pre-commit run --all-files passes
• Local tox run is green

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.0%. Comparing base (cb28440) to head (129d2b8).

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #70   +/-   ##
=====================================
  Coverage   90.0%   90.0%           
=====================================
  Files          3       3           
  Lines         30      30           
  Branches       1       1           
=====================================
  Hits          27      27           
  Misses         2       2           
  Partials       1       1           

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