Vendor omniauth-cas3 gem

* Our ultimate goal is to upgrade `omniauth` to v2.0. See the related
  [issue](https://gitlab.com/gitlab-org/gitlab/-/issues/30073)
* In order to upgrade `omniauth` to v2.0, we need to upgrade
  `omniauth-cas3` because the latter has a dependency on the former.
* There is an open issue asking the maintainer to make `omniauth-cas3`
  compatible with `omniauth` v2.0 but it has not been responded to:
  https://github.com/tduehr/omniauth-cas3/issues/5
* This MR vendors the gem and updates its `.gemspec` file so that it is
  compatible with `omniauth` v2.0
* We may deprecate our usage of this gem in the future
  [(issue)](https://gitlab.com/gitlab-org/gitlab/-/issues/366212)
* MR where we vendored a different omniauth gem for same reasons:
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/92485