use Gitlab::UserSettings directly as a singleton instead of including/extending it

What does this MR do?

Allows Gitlab::CurrentSettings#current_application_settings keys to be called directly on the Gitlab::CurrentSettings module via method_missing.

Removes all inclusions and extensions of said module and replaces all calls to current_application_settings with a direct call to the setting name (ex. current_application_settings.default_project_creation becomes Gitlab::CurrentSettings.default_project_creation)

Are there points in the code the reviewer needs to double check?

#current_application_settings shouldn't be used bare anywhere anymore

Why was this MR needed?

tech debt repayment :) (see #37698 (closed) and !13955 (merged))

Closes #37698 (closed)