CompatHelper: add new compat entry for ModelingToolkit at version 9 for package docs, (keep existing compat)

[github-actions[bot]]

This pull request sets the compat entry for the ModelingToolkit package to 9 for package docs. 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. Note: Consider registering a new release of your package immediately after merging this PR, as downstream packages may depend on this for tests to pass.

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