updated algorithm from 128 to 256 bit

It's 2021, shouldn't we all be using 256 as a minimum by now? I think we should set the example in the documentation.

Author: sivey42

docs/relational-databases/security/encryption/transparent-data-encryption.md

@@ -103,7 +103,7 @@ go
 USE AdventureWorks2012;
 GO
 CREATE DATABASE ENCRYPTION KEY
-WITH ALGORITHM = AES_128
+WITH ALGORITHM = AES_256
 ENCRYPTION BY SERVER CERTIFICATE MyServerCert;
 GO
 ALTER DATABASE AdventureWorks2012