CompatHelper: bump compat for JuliaFormatter to 0.20, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the JuliaFormatter package from ^0.4.3, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 0.12, 0.13 to ^0.4.3, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 0.12, 0.13, 0.20. 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.