CompatHelper: bump compat for ModelingToolkit to 9 for package StiffODE, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the ModelingToolkit package from 8 to 8, 9 for package StiffODE. 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.