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

[github-actions[bot]]

This pull request changes the compat entry for the NLPModels package from 0.20 to 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.