Add Wagtail 7.0 to test matrix

[Stormheg]

Stop testing against unsupported versions of Wagtail and Django in an
attempt to keep testing matrix combinations under control.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.78%. Comparing base (fe8852f) to head (00dc7a9).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #97   +/-   ##
=======================================
  Coverage   93.78%   93.78%           
=======================================
  Files          43       43           
  Lines        1690     1690           
  Branches      111      111           
=======================================
  Hits         1585     1585           
  Misses         69       69           
  Partials       36       36           

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