Define Requirements for PMI on Drupal 9

We need to define requirements for the Drupal 9 implementation of the PMI.

Such as:

• What Content types do we keep (started - just need clarification for MailMessage)
• What Taxonomy vocabulary/terms do we keep (started)
• What Views do we keep (started)
• What API endpoints do we keep
• What modules do we keep
• What functionality from the pmi module do we keep
• What functionality from the eclipse_api module do we keep
• What user roles do we keep

I will be posting comments for each of these items down below so we can discuss what we keep/remove from the D9 PMI implementation.

Also let me know if I missed an important element that should be added to the list above.

/cc @wbeaton @cguindon