Remove encryption service and related configurations

The `IEncryptionService` interface and its implementation,
`DataProtectionEncryptionService`, have been removed from the
codebase. These components previously provided encryption and
decryption functionality using the ASP.NET Core Data Protection API.

Additionally, the dependency injection setup for the encryption
service and the Data Protection API configuration (e.g., key
persistence and application name) have been removed from
`DependencyInjection.cs`.

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IEncryptionService.cs

@@ -1,10 +0,0 @@
-namespace DigitalData.MessagingService.Application.Common.Interfaces;
-
-/// <summary>
-/// Encryption service interface for password encryption.
-/// </summary>
-public interface IEncryptionService
-{
-    string Encrypt(string plainText);
-    string Decrypt(string cipherText);
-}