CompatHelper: bump compat for MixedModels to 5, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the MixedModels package from 4 to 4, 5.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.76%. Comparing base (399bcb9) to head (7b5d2c9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #79   +/-   ##
=======================================
  Coverage   95.76%   95.76%           
=======================================
  Files           3        3           
  Lines         118      118           
=======================================
  Hits          113      113           
  Misses          5        5           

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