Docs: SAML external groups are supported in free edition

What does this MR do?

Update docs to reflect that SAML external groups are supported in free edition based on https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/auth/saml/user_spec.rb#L48-79, https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/auth/saml/user.rb#L26, & https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/auth/saml/config.rb#L20-22 as well as lack of any external details in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/auth/saml/config.rb or https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/auth/saml/user.rb.

Related issues

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