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

appveyor.sh

@@ -66,7 +66,6 @@ if [ "${BUILD_SYSTEM}" = 'CMake' ]; then
     [ "${_chkprefill}" = '_chkprefill' ] && options+=' -D_CURL_PREFILL=OFF'
     [[ "${TARGET}" = *'ARM64'* ]] && SKIP_RUN='ARM64 architecture'
     [ -n "${TOOLSET:-}" ] && options+=" -T ${TOOLSET}"
-    [ -n "${WINTARGET:-}" ] && options+=" -DCURL_TARGET_WINDOWS_VERSION=${WINTARGET}"
     [ "${OPENSSL}" = 'ON' ] && options+=" -DOPENSSL_ROOT_DIR=${openssl_root_win}"
     [ -n "${CURLDEBUG:-}" ] && options+=" -DENABLE_CURLDEBUG=${CURLDEBUG}"
     if [ "${APPVEYOR_BUILD_WORKER_IMAGE}" = 'Visual Studio 2013' ]; then