Error code should be string in HttpExceptionNormalizer

Problem/Motivation

JSON:API error code should be string as stated in specification and json schema.

https://jsonapi.org/format/#errors

code: An application-specific error code, expressed as a string value.

Here is the provided example, which shows the code as a string and not an integer in the JSON response

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/vnd.api+json

{
  "jsonapi": { "version": "1.0" },
  "errors": [
    {
      "code":   "123",
      "source": { "pointer": "/data/attributes/firstName" },
      "title":  "Value is too short",
      "detail": "First name must contain at least three characters."
    },
    {
      "code":   "225",
      "source": { "pointer": "/data/attributes/password" },
      "title": "Passwords must contain a letter, number, and punctuation character.",
      "detail": "The password provided is missing a punctuation character."
    },
    {
      "code":   "226",
      "source": { "pointer": "/data/attributes/password" },
      "title": "Password and password confirmation do not match."
    }
  ]
}

Proposed resolution

Cast the error code as string

Remaining tasks

Patch

User interface changes

N/A

API changes

N/A

Data model changes

Release notes snippet

JSON:API now provides HTTP error codes as strings instead of integers in order to comply with the specification. If your client application relies on an integer data type for an HTTP error code, it may need to be updated.