Vendor omniauth-azure-oauth2 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-azure-oauth2` because the latter has a dependency on the
former.
* There is an open issue asking the maintainer to make
  `omniauth-azure-oauth2` compatible with `omniauth` v2.0 but the author
  advised forking the gem:
  https://github.com/marknadig/omniauth-azure-oauth2/pull/29#issuecomment-663113582
* There is a fork called `omniauth-azure-activedirectory-v2` that
  supports [*only* the 2.0
  endpoints](https://github.com/RIPAGlobal/omniauth-azure-activedirectory-v2/pull/1)
  for Active Directory but we need to keep using `omniauth-azure-oauth2`
  for our customers who use the 1.0 ActiveDirectory endpoints
* 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