CompatHelper: bump compat for RDatasets in [extras] to 0.8, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the RDatasets package from 0.5, 0.6, 0.7 to 0.5, 0.6, 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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.42%. Comparing base (23a4e20) to head (4d7da6f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #616   +/-   ##
=======================================
  Coverage   95.42%   95.42%           
=======================================
  Files           8        8           
  Lines        1006     1006           
=======================================
  Hits          960      960           
  Misses         46       46           

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