Clean up some old cruft related to Windows

Hi all,

As discussed here, there is in the tree a couple of things related to
past versions of Windows:
https://www.postgresql.org/message-id/20191218021954.GE1836@paquier.xyz

So I have been looking at that more closely, and found more:
- MIN_WINNT can be removed from win32.h thanks to d9dd406 which has
added a requirement on C99 with Windows 7 as minimum platform
supported.  (The issue mentioned previously.)
- pipe_read_line(), used when finding another binary for a given
installation via find_other_exec() has some special handling related
to Windows 2000 and older versions.
- When trying to load getaddrinfo(), we try to load it from
wship6.ddl, which was something needed in Windows 2000, but newer
Windows versions include it in ws2_32.dll.
- A portion of the docs still refer to Windows 98.

Thoughts?
--
Michael