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.h

@@ -76,21 +76,6 @@ CURLcode Curl_socket_open(struct Curl_easy *data,
 int Curl_socket_close(struct Curl_easy *data, struct connectdata *conn,
                       curl_socket_t 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
-
-*/
-void Curl_sndbuf_init(curl_socket_t sockfd);
-#else
-#define Curl_sndbuf_init(y) Curl_nop_stmt
-#endif
-
 /**
  * Creates a cfilter that opens a TCP socket to the given address
  * when calling its `connect` implementation.