Deprecate user service interfaces in controllers

Added [Obsolete] attributes to _userService in AuthController,
_repository in EmailTemplateController, and _userRepository
in EnvelopeExecutor. These changes guide developers to
transition to using MediatR and IRepository.

EnvelopeGenerator.GeneratorAPI/Controllers/AuthController.cs

@@ -16,6 +16,7 @@ namespace EnvelopeGenerator.GeneratorAPI.Controllers;
 public partial class AuthController : ControllerBase
 {
     private readonly ILogger<AuthController> _logger;
+    [Obsolete("Use MediatR")]
     private readonly IUserService _userService;
     private readonly IDirectorySearchService _dirSearchService;
 
@@ -25,6 +26,7 @@ public partial class AuthController : ControllerBase
     /// <param name="logger">The logger instance.</param>
     /// <param name="userService">The user service instance.</param>
     /// <param name="dirSearchService">The directory search service instance.</param>
+    [Obsolete("Use MediatR")]
     public AuthController(ILogger<AuthController> logger, IUserService userService, IDirectorySearchService dirSearchService)
     {
         _logger = logger;