Prepare the backend code for instance level security dashboard settings page

What does this MR do?

Related to #230413 (closed)

This is the first part of an MR that will be split into two pieces. It simply prepares the backend code to serve the instance dashboard settings page.

The sidebar is commented out intentionally, it will be activated in the next MR which is going to be purely a frontend MR.

Screenshots

There are no visual changes to the user.

Edit

Here is the second MR just in case more context is needed: !38965 (merged)

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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