Prep for Wagtail 7.0

[zerolab]

• updates the test matrix and adds the Wagtail 7 PyPI trove classifier
• bumps ruff
• uses SPDX license expressions in pyproject.toml
• updates changelog

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.26%. Comparing base (345786a) to head (0edb0b0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #864   +/-   ##
=======================================
  Coverage   93.26%   93.26%           
=======================================
  Files          47       47           
  Lines        4249     4249           
  Branches      552      552           
=======================================
  Hits         3963     3963           
  Misses        171      171           
  Partials      115      115           

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