CompatHelper: bump compat for Rmath to 0.8, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Rmath package from 0.6.1, 0.7 to 0.6.1, 0.7, 0.8.
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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.22%. Comparing base (d8ed2dc) to head (ace560f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   63.22%   63.22%           
=======================================
  Files          14       14           
  Lines         639      639           
=======================================
  Hits          404      404           
  Misses        235      235           

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