CompatHelper: bump compat for NLPModels to 0.21, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the NLPModels package from ~0.17.2, 0.18, 0.19, 0.20 to ~0.17.2, 0.18, 0.19, 0.20, 0.21.
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]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.17%. Comparing base (2e301da) to head (752328f).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #331   +/-   ##
=======================================
  Coverage   70.17%   70.17%           
=======================================
  Files          45       45           
  Lines        3943     3943           
=======================================
  Hits         2767     2767           
  Misses       1176     1176           

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