[2/4] Deprecate api_user field in auth0_email resource

[sergiught]

🔧 Changes

We're deprecating the APIUser field because this field is no longer supported and trying to include it in the payload will result in a 400:

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": "Payload validation error: 'None of the valid schemas were met' on property credentials (Credentials required to use the provider). Inner errors: [ Payload validation error: 'Additional properties not allowed: api_user' on property credentials ({description}). ].",
  "errorCode": "invalid_body"
}

📚 References

🔬 Testing

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)