Fix HTTP verb in user approval API

What does this MR do?

Fix HTTP verb for 201 to be Created rather than 201 OK

Disclaimer I'm aware that this is an odd change. Ideally we would use 200 for an OK here, but the API renders 201 on success for this particular endpoint. This change is to ensure clarity and avoid confusion to those using this endpoint, and seeing 201 CREATED in the request, but expect 201 OK.

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.