Swap Grape over to Gitlab::Json

What does this MR do?

Overrides how Grape generates JSON. By default Grape uses ActiveSupport .to_json handling if available or then swaps to MultiJson. This makes it impossible to ensure Oj is always used for rendering API responses if desired, so we need to override the formatter and ensure it uses Gitlab::Json instead.

Related #217633 (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
• 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