[PM-29556] Fix: changing organization plan nulls out public and private keys (#6738)

Main fix: only assign new key value where old keys are not set
and new keys have been provided.

Refactors:
- use consistent DTO model for keypairs
- delete duplicate property assignment for new orgs

src/Api/AdminConsole/Models/Request/Organizations/OrganizationUpdateRequestModel.cs

@@ -22,7 +22,6 @@ public class OrganizationUpdateRequestModel
         OrganizationId = organizationId,
         Name = Name,
         BillingEmail = BillingEmail,
-        PublicKey = Keys?.PublicKey,
-        EncryptedPrivateKey = Keys?.EncryptedPrivateKey
+        Keys = Keys?.ToPublicKeyEncryptionKeyPairData()
     };
 }