Update doorkeeper and doorkeeper-openid_connect gems

This is needed to prepare for adding PKCE support in
https://gitlab.com/gitlab-org/gitlab/-/issues/382340.

https://github.com/doorkeeper-gem/doorkeeper-openid_connect/pull/173
modified the initialization so that
`app/controllers/concerns/enforces_two_factor_authentication.rb` is
loaded before the Rails application is initialized, so we move the
`MFA_HELP_PAGE` constant into a method to avoid boot errors.

Diffs:
- https://my.diffend.io/gems/doorkeeper/5.5.0.rc2/5.5.4
- https://my.diffend.io/gems/doorkeeper-openid_connect/1.7.5/1.8.2

Changelog: changed