Add Auth0, JWT and Shibboleth to list of providers with icons

What does this MR do and why?

GitLab has a provider icon for Auth0, JWT and Shibboleth but they are not displayed on the sign-in or registration page when the providers are enabled. This MR corrects this by adding them to the list of providers with icons.

Icons for reference:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/images/auth_buttons/auth0_64.png
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/images/auth_buttons/jwt_64.png
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/images/auth_buttons/shibboleth_64.png

Screenshots or screen recordings

Context	Before	After
Sign-in
Registration

How to set up and validate locally

1. Add the Auth0, JWT and Shibboleth providers to the GitLab config. Full configuration is not necessary for validation. Example gitlab.yml entry:

  omniauth:
    providers:
    - { name: 'auth0' }
    - { name: 'jwt' }
    - { name: 'shibboleth' }

2. Validate the buttons appear on the sign-in and registration pages with their provider icons

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.