Mark Auth record as obsolete with replacement guidance

The `Auth` record in the `EnvelopeGenerator.Server.Models`
namespace has been marked as `[Obsolete]` with the message
"Use auth DTO" to indicate it is outdated and should be
replaced with a newer implementation.

The `Auth` record includes the following properties:
- `AccessCode`, `SmsCode`, `AuthenticatorCode` (nullable strings)
- `UserSelectSMS` (boolean)

Additionally, it defines computed properties `HasAccessCode`
and `HasSmsCode` to check for the presence of `AccessCode`
and `SmsCode`, respectively.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Models/Auth.cs

@@ -1,5 +1,6 @@
 namespace EnvelopeGenerator.Server.Models;
 
+[Obsolete("Use auth DTO")]
 public record Auth(string? AccessCode = null, string? SmsCode = null, string? AuthenticatorCode = null, bool UserSelectSMS = default)
 {
     public bool HasAccessCode => AccessCode is not null;