Don't return warnings as errors in sqlsrv driver

The SQL Server platforms use `sp_rename` stored procedure for renaming schema objects like tables, indexes or columns. SQL Server raises a warning when invoking this stored procedure, although the result is as expected:

```php
Doctrine\DBAL\Driver\SQLSrv\SQLSrvException: SQLSTATE [01000, 15477]: [Microsoft][SQL Server Native Client 11.0][SQL Server]Vorsicht: Wenn Sie Teile eines Objektnamens ändern, werden Skripts und gespeicherte Prozeduren möglicherweise funktionsunfähig.
```

This PR configures the sqlsrv driver not to handle warnings returned from the server as errors to prevent this. The PDO_SQLSRV driver silences warnings, too.