windows: bump minimum to Vista (from XP)

After this patch curl requires targeting Vista or newer, and a toolchain
with Vista support.

Supported MSVC compilers (VS2010+) all support Vista:
- VS2012+ target Win8 (or later) by default.
- VS2010 targets Win7 by default.

Supported mingw-w64 versions (v3+) all support Vista:
- mingw-w64 v9+ target Win10 by default.
- mingw-w64 v8 and older target Server 2003 (~XP) by default.
  After this patch it may be necessary to override the default Windows
  target version to Vista (or newer) via:
  autotools: `CPPFLAGS=-D_WIN32_WINNT=0x0600`
  cmake: `-DCURL_TARGET_WINDOWS_VERSION=0x0600`
- mingw-w64 v6+ allow changing the default at toolchain build-time.

Notes:
- For non-MSVC, non-mingw-w64 toolchains, `if_nametoindex` needs to be
  allowlisted in `curl_setup.h`, if they do support it.

Fixes #17985 (discussion)
Closes #18009

lib/cf-socket.c

@@ -440,37 +440,6 @@ int Curl_socket_close(struct Curl_easy *data, struct connectdata *conn,
   return socket_close(data, conn, FALSE, sock);
 }
 
-#ifdef USE_WINSOCK
-/* When you run a program that uses the Windows Sockets API, you may
-   experience slow performance when you copy data to a TCP server.
-
-   https://learn.microsoft.com/troubleshoot/windows-server/networking/slow-performance-copy-data-tcp-server-sockets-api
-
-   Work-around: Make the Socket Send Buffer Size Larger Than the Program Send
-   Buffer Size
-
-   The problem described in this knowledge-base is applied only to pre-Vista
-   Windows. Following function trying to detect OS version and skips
-   SO_SNDBUF adjustment for Windows Vista and above.
-*/
-
-void Curl_sndbuf_init(curl_socket_t sockfd)
-{
-  int val = CURL_MAX_WRITE_SIZE + 32;
-  int curval = 0;
-  int curlen = sizeof(curval);
-
-  if(Curl_isVistaOrGreater)
-    return;
-
-  if(getsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, (char *)&curval, &curlen) == 0)
-    if(curval > val)
-      return;
-
-  setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, (const char *)&val, sizeof(val));
-}
-#endif /* USE_WINSOCK */
-
 /*
  * Curl_parse_interface()
  *
@@ -1133,8 +1102,6 @@ static CURLcode cf_socket_open(struct Curl_cfilter *cf,
 
   nosigpipe(cf, data, ctx->sock);
 
-  Curl_sndbuf_init(ctx->sock);
-
   if(is_tcp && data->set.tcp_keepalive)
     tcpkeepalive(cf, data, ctx->sock);