SSO-provisioned users redirect to provider login after confirmation

What does this MR do and why?

• Previously, all users were redirected to regular sign in page after confirming their email address via the emailed link.
• This would lead to a poor onboarding experience.
• Follow-on MR should also update where users are redirected if their account is already confirmed but this is a first iteration to fix the issue for the majority of users.
• Related issue: #298753 (closed)

Screenshots or screen recordings

This is what I see as an SSO-provisioned user after clicking on the confirmation link in my email:

How to set up and validate locally

1. Set up group SAML https://docs.gitlab.com/ee/user/group/saml_sso/ (if you need to set it up for the first time, Okta is probably the easiest but there are also instructions on how to set up group SAML with Docker in GDK)
2. Sign into GitLab via SSO for a brand new user
3. Click on the link in the email with subject "Confirm your email address" in https://gdk.test:3443/rails/letter_opener
4. You should be taken to the SSO login page for the group with Group SAML configured

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.