CompatHelper: add new compat entry for BetaML at version 0.9, (keep existing compat)

[github-actions[bot]]

This pull request sets the compat entry for the BetaML package to 0.9. 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.