CompatHelper: add new compat entry for Unitful at version 1, (keep existing compat)

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This pull request sets the compat entry for the Unitful package to 1. 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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1 out of 2 committers have signed the CLA.
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