net: remove sysSocket fallback for Windows 7

net: remove sysSocket fallback for Windows 7

`syscall.ForkLock` is not used in `syscall.StartProcess` since
CL 288297, so using it in `sysSocket` makes no sense. This CL
goes a little bit further and removes the `sysSocket` fallback for
Windows 7, since it is not supported since go 1.21 (#57003).

Updates #60942

Change-Id: If14a0d94742f1b80af994f9f69938701ee41b402
Reviewed-on: https://go-review.googlesource.com/c/go/+/506136
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>