CompatHelper: bump compat for "JuMP" to "0.21"

[github-actions[bot]]

This pull request changes the compat entry for the JuMP package from ~0.18.0 to ~0.18.0, 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.