Fix MSSQL's default username

[Eng-Fouad]

Fixes #7237

If MSSQL_COLLATION is set to a collation with case-sensetive (_CS not _CI), the username SA will fail to login to DB, as sa is the correct username.

[eddumelendez]

According to the docs, the username is sa. Also, the default collation is SQL_Latin1_General_CP1_CI_AS which is case insensitive, performing SELECT SERVERPROPERTY('COLLATION').

[eddumelendez]

Thanks for your contribution, @Eng-Fouad !