Re-enable TCP keepalive support on Windows
The current version of XMRig has TCP keepalive support disabled for Windows: https://github.com/xmrig/xmrig/commit/91ed7e36cd109714ed2c7bdace4986984a7ad34e

The commit message is from 6 years ago and cites TCP keepalive behaviour (via libuv) not working as intended/designed.  This is quite understandable given [how XP/2K behaved](https://learn.microsoft.com/en-us/windows/win32/winsock/so-keepalive).

However, we're now in 2023, and Windows 7 is the "oldest" OS commonly around the Internet now.  I myself only switched from W7 to W10 this year.

Would it be possible to re-enable libuv TCP keepalive support, or possibly change the `#ifndef` to utilise [WINVER/other defines](https://learn.microsoft.com/en-us/cpp/porting/modifying-winver-and-win32-winnt?view=msvc-170) to ensure the feature is only enabled on Windows 7 or newer?  This might only work on MSVC builds (unsure about gcc).

If XMRig devs still want to support Windows XP (that's fine!), then we can close this ticket.

Otherwise, as for testing: the only pool I know which supports TCP keepalives for certain is Zergpool when used alongside their TLS/SSL endpoints.  Their TLS proxy (haproxy) uses `option clitcpka` ([documentation](https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#4-option%20clitcpka)).  I know because I've been helping them get TLS/SSL working again in recent days and been doing a bit of packet captures.  Unsure if their plaintext endpoints support it or not.