HTTP status code for "NOT READY"

[ivarconr]

Describe the bug

Today when edge is not considered ready you get a json response explaining it is not ready with a 200 status code. I think it would be more appropriate to use 503 Service Unavailable.

Steps to reproduce the bug

1. start edge (with invalid url or token)
2. curl ready endpoint: curl localhost:3063/internal-backstage/ready
3. inspect response:

   
   HTTP/1.1 200 OK
   content-encoding: gzip
   content-type: application/json
   date: Wed, 08 May 2024 18:21:25 GMT
   etag: W/"15-NytVk1xF3R-LK3QpS5Swfg=="
   transfer-encoding: chunked
   vary: accept-encoding, Origin, Access-Control-Request-Method, Access-Control-Request-Headers

{ "status": "NOTREADY" }

### Expected behavior

Use status code  [503 Service Unavailable](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/503) instead of "200". 

### Logs, error output, etc.

_No response_

### Screenshots

_No response_

### Additional context

_No response_

### Unleash version

_No response_

### Subscription type

None

### Hosting type

None

### SDK information (language and version)

_No response_

[ivarconr]

solved in 19.1.2