CompatHelper: bump compat for ApproxFun to 0.13 for package MOLPDE, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the ApproxFun package from 0.11, 0.12 to 0.11, 0.12, 0.13 for package MOLPDE. 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.