Remove support for old handshake

Currently, the library includes the following code to handle an old version of the client handshake https://github.com/jonhoo/msql-srv/blob/26a5c35ed648f31cd2bb3943806e11b61b9dfd90/src/commands.rs#L47-L63

This gives the impression that the library supports that version of the protocol, but that is probably not the case. Since the old handshake was [replaced in version 3.21.0](https://dev.mysql.com/doc/internals/en/connection-phase-packets.html#packet-Protocol::Handshake), released in 1998, I think it is safe to assume no MySQL client out there will be using it. IMO it would make sense to remove it from the codebase, both to simplify the code and to avoid confusion for people less familiar with the MySQL protocol.

Start by reading src/commands.rs around lines 47-63 and the linked MySQL handshake documentation to understand the legacy path. Remove the obsolete handshake support while preserving the current protocol handling, then run the repository's existing tests to confirm the change.

Written by the indexing model from the issue text.