CompatHelper: bump compat for LinearSolve to 4, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the LinearSolve package from 2, 3 to 2, 3, 4.
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 72.05%. Comparing base (08d2035) to head (ecba5ed).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #70   +/-   ##
=======================================
  Coverage   72.05%   72.05%           
=======================================
  Files          10       10           
  Lines         340      340           
=======================================
  Hits          245      245           
  Misses         95       95           

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