Allow Minimal Access role for top-level SAML Group Links

What does this MR do and why?

Fixes #334577 (closed)

This allows customers to specify SAML Group Links with a minimal access role only for the top-level groups.

We allow minimal access as the default role for SAML users in top-level groups, but previously did not allow specifying SAML Group Links with a minimal access role. This presents a problem for customers that wish to use the minimal access role because if a matching SAML Group Link is not added, the users will be removed from the top-level group and get a 404 for the entire hierarchy.

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