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 0.18, 0.19, 0.20 to 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.

[github-actions]

Package name	latest	stable
NLSProblems.jl
OptimizationProblems.jl

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.00%. Comparing base (8dd876f) to head (8ae962b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
- Coverage   93.20%   93.00%   -0.20%     
==========================================
  Files           5        4       -1     
  Lines         662      629      -33     
==========================================
- Hits          617      585      -32     
+ Misses         45       44       -1     

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