Fix MSSQL's default username (#7238)

According to the [docs](https://hub.docker.com/_/microsoft-mssql-server), the username is `sa`. 
Default collation is `SQL_Latin1_General_CP1_CI_AS` which is case insensitive.

Author: Eng-Fouad

modules/mssqlserver/src/main/java/org/testcontainers/containers/MSSQLServerContainer.java

@@ -20,7 +20,7 @@ public class MSSQLServerContainer<SELF extends MSSQLServerContainer<SELF>> exten
 
     public static final Integer MS_SQL_SERVER_PORT = 1433;
 
-    static final String DEFAULT_USER = "SA";
+    static final String DEFAULT_USER = "sa";
 
     static final String DEFAULT_PASSWORD = "A_Str0ng_Required_Password";