Update doorkeeper and doorkeeper-openid_connect gems

What does this MR do and why?

This is needed to prepare for adding PKCE support in #382340 (closed).

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:

% bin/rake db:migrate
rake aborted!
NoMethodError: undefined method `help_page_url' for #<Module:0x00000001274c0158>
/Users/stanhu/gdk-ee/gitlab/app/controllers/concerns/enforces_two_factor_authentication.rb:13:in `<module:EnforcesTwoFactorAuthentication>'
/Users/stanhu/gdk-ee/gitlab/app/controllers/concerns/enforces_two_factor_authentication.rb:10:in `<main>'
/Users/stanhu/gdk-ee/gitlab/lib/gitlab/base_doorkeeper_controller.rb:9:in `<class:BaseDoorkeeperController>'
/Users/stanhu/gdk-ee/gitlab/lib/gitlab/base_doorkeeper_controller.rb:7:in `<module:Gitlab>'
/Users/stanhu/gdk-ee/gitlab/lib/gitlab/base_doorkeeper_controller.rb:5:in `<main>'
/Users/stanhu/github/doorkeeper-openid_connect/lib/doorkeeper/openid_connect/engine.rb:11:in `block in <class:Engine>'
/Users/stanhu/gdk-ee/gitlab/config/environment.rb:7:in `<main>'
Tasks: TOP => db:migrate => db:load_config => environment
(See full trace by running task with --trace)

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

It seems this update brings in https://gitlab.example.com/.well-known/oauth-authorization-server as a route, which appears to address #233956 (closed).

MR acceptance checklist

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