Update vendored omniauth-cas3 for OmniAuth 2.0

What does this MR do and why?

• OmniAuth v2 just around the corner: https://gitlab.com/gitlab-org/gitlab/-/issues/30073
• When I upgraded the dependency for omniauth-cas3 in a draft MR, some specs started breaking.
• OmniAuth v2 converts all GET requests to POST requests by default with CSRF protection.
• This approach copied over from changes made to a different omniauth-cas gem: https://github.com/dlindahl/omniauth-cas/pull/68/files

Screenshots or screen recordings

I confirmed locally that the specs for this gem now pass when running them with OmniAuth v2. The OmniAuth v2 upgrade will happen in a separate MR.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.