docs: update custom stream SOCKS5 example to modern API (fixes outdated callback usage)

[Priyank-Gaur]

This PR updates the “Connecting using custom stream” documentation.

The old example used a callback-style:
stream: function (cb) { ... }

This pattern no longer works in current versions of mysql2 and results in the runtime error:
TypeError: cb is not a function

What changed

Replaced the outdated callback-based SOCKS5 example.
Added a modern example where stream returns a Duplex stream or a Promise that resolves to one.
Added a short note explaining that the callback-style API is no longer supported.

Why this change

The existing docs mislead users into using a deprecated pattern that fails on current mysql2 versions.
The updated version shows the correct API and prevents confusion/errors for anyone connecting over SOCKS5 or other custom transport streams.

Related Issue
Fixes #3655

[wellwelwel]

Thanks, @Priyank-Gaur! ✨

[siddharthvp]

What versions of mysql2 and socks is this tested with?

With the latest ones (mysql2 3.15.3, socks 2.8.7):

Firstly, socks createConnection requires command to be passed. Without that it gives:

SocksClientError: An invalid SOCKS command was provided. Valid options are connect, bind, and associate.

If we set command to any of those 3 values, the code gives:

    this.stream.on('error', this._handleNetworkError.bind(this));
                ^
TypeError: this.stream.on is not a function

when getting a connection from the pool.