Resolve "User admin approval - Enable/disable toggle `require_admin_approval_after_user_signup` via API"

What does this MR do?

For #263106 (closed)

This change:

• Allows setting the value of require_admin_approval_after_user_signup to a Boolean via the PUT /application/settings API.

• Exposes the value of require_admin_approval_after_user_signup via the GET /application/settings API (This functionality exists even today because how our Settings API & the Admin UI uses the same helpers, but this was never documented before - now this is documented in this MR).

Screenshots (strongly suggested)

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