Mark CreateAsync as obsolete; update Flag reference

Marked CreateAsync in ReadOnlyController as [Obsolete] to encourage use of MediatR. Simplified Flag.DataIntegrityIssue reference in EnvelopeMailService error handling.

EnvelopeGenerator.API/Controllers/ReadOnlyController.cs

@@ -38,6 +38,7 @@ public class ReadOnlyController : ControllerBase
     /// <param name="createDto">Creation payload.</param>
     [HttpPost]
     [Authorize(Policy = AuthPolicy.Receiver)]
+    [Obsolete("Use MediatR")]
     public async Task<IActionResult> CreateAsync([FromBody] EnvelopeReceiverReadOnlyCreateDto createDto)
     {
         var authReceiverMail = User.GetReceiverMailOfReceiver();