fix: Use UUID as issuer in SETs

docs/receiving/process-and-acknowledge.mdx

@@ -49,6 +49,7 @@ Sofern Sie den Fehler "'iat' must not be after ..." (`https://schema.fitko.de/fi
   ```java title=Annahmen
   JWK signaturePrivateKey; // privater (geheimer) Schlüssel des für die Signaturerstellung/-prüfung genutzten Schlüsselpaares des empfangenden Systems
   JWK signaturePublicKey;  // öffentlicher Schlüssel des für die Signaturerstellung/-prüfung genutzten Schlüsselpaares des empfangenden Systems
+  String issuer = "943da3e2-c178-4b63-aa2d-2fe964b0e09d"; // our own destinationId (MUST match destinationId of this submission)
   String subject = "submission:f65feab2-4883-4dff-85fb-169448545d9f"; // submissionId
   String event ="https://schema.fitko.de/fit-connect/events/accept-submission";
   String transactionId = "case:f73d30c6-8894-4444-8687-00ae756fea90"; // caseId
@@ -61,7 +62,7 @@ Sofern Sie den Fehler "'iat' must not be after ..." (`https://schema.fitko.de/fi
   try {
     JWSSigner signer = new RSASSASigner(signaturePrivateKey.toRSAKey());
     JWTClaimsSet claimsSet = new JWTClaimsSet.Builder()
-            .issuer("https://my-custom-identifiable-service.domain")
+            .issuer(issuer)
             .issueTime(new Date())
             .jwtID(UUID.randomUUID().toString())
             .subject(subject)