Restrict duo pro assignment email to duo pro for sm

What does this MR do and why?

Restrict duo pro assignment email to duo pro fo... (!166738 - merged) is being backported

Duo Pro seat assignment notification email for self-managed was added in %17.4 release: !164104 (merged). But we should guard the email to only Duo Pro and not Duo Enterprise, since the email content is Duo Pro specific.

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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