Use RolesFinder for populating role selector

What does this MR do and why?

Find member roles using the MemberRoles::RolesFinder instead of directly querying the root group.

This allows instance level custom roles to be assigned as SAML default role or for SAML Group Links.

Issue: #439910 (closed)

How to set up and validate locally

1. Make sure SaaS mode is turned off
2. Enable group SAML

   gdk config set omniauth.group_saml.enabled true && gdk reconfigure && gdk restart rails-web

3. Create a group and apply the Ultimate license
4. Visit http://localhost:3000/admin/application_settings/roles_and_permissions and create an instance level custom role
5. Visit http://localhost:3000/groups/${new_group}/-/saml and verify the new custom role is an option in the dropdown
6. Visit http://localhost:3000/groups/${new_group/-/saml_group_links and verify the new custom role is an option in the dropdown