Reapply "refactor(RSACryptographer): Entfernte nullbare Eigenschaft von Issuer und Audience."

This reverts commit 600d17ef40.

DigitalData.Core.Security/Cryptographer/RSAEncryptor.cs

@@ -5,7 +5,7 @@ namespace DigitalData.Core.Security.Cryptographer
 {
     public class RSAEncryptor : RSACryptographer, IRSAEncryptor, IRSACryptographer
     {
-        public override required string Pem
+        public override string Pem
         {
             get => base.Pem;
             init
@@ -14,7 +14,7 @@ namespace DigitalData.Core.Security.Cryptographer
                 base.Pem = value;
             }
         }
-
+        
         public byte[] Encrypt(byte[] data) => RSA.Encrypt(data, Padding);
 
         public string Encrypt(string data) => RSA.Encrypt(data.Base64ToByte(), Padding).BytesToString();