Add admin mode controller path to Rack::Attack defaults

What does this MR do?

In !18214 (merged) the admin mode feature was introduced, with a new controller endpoint /admin/session. This adds the admin mode controller endpoint to Rack::Attack defaults.

The Omnibus configuration for Rack::Attack is deprecated, so the configuration is added to the application settings in the db.

🛠 with ❤ at Siemens (/cc @bufferoverflow)

Replaces omnibus-gitlab!3758 (closed)

Related to #36872 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team