Fix tds_version _mssql connection property value for TDS > 7.

Use the same numbering strategy as MS: There isn't TDS version 8.0 nor
9.0 but 7.1 and 7.2 respectively.

This is backward incompatible with user code that tests for value 8.0
when using TDS version 7.1.

Refs 32584060 and a8f58fa1.