Rename rack_attack.rb initializer

RackAttack config file was renamed to 'rack_attack_new.rb' so it was
able to co-exists with the Omnibus settings (for more info see
https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/31246#note_215110463)

Since the Omnibus RackAttack settings were removed on 13.0, we can
rename this initializer back to its original name.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/218291

doc/security/rack_attack.md

@@ -121,9 +121,6 @@ The following settings can be configured:
 
 **Installations from source**
 
-NOTE: **Note:** Rack Attack initializer was temporarily renamed to `rack_attack_new`, to
-support backwards compatibility with the one [Omnibus initializer](https://docs.gitlab.com/omnibus/settings/configuration.html#setting-up-paths-to-be-protected-by-rack-attack). It'll be renamed back to `rack_attack.rb` once Omnibus throttle is removed. Please see the [GitLab issue](https://gitlab.com/gitlab-org/gitlab/-/issues/29952) for more information.
-
 These settings can be found in `config/initializers/rack_attack.rb`. If you are
 missing `config/initializers/rack_attack.rb`, the following steps need to be
 taken in order to enable protection for your GitLab instance: