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/ftp.c

@@ -583,9 +583,6 @@ static CURLcode ftp_initiate_transfer(struct Curl_easy *data,
        size prior to the actual upload. */
     Curl_pgrsSetUploadSize(data, data->state.infilesize);
 
-    /* set the SO_SNDBUF for the secondary socket for those who need it */
-    Curl_sndbuf_init(data->conn->sock[SECONDARYSOCKET]);
-
     /* FTP upload, shutdown DATA, ignore shutdown errors, as we rely
      * on the server response on the CONTROL connection. */
     Curl_xfer_setup_send(data, SECONDARYSOCKET);