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.yml

@@ -61,11 +61,10 @@ environment:
       SCHANNEL: 'ON'
       DEBUG: 'OFF'
       CURLDEBUG: 'ON'
-    - job_name: 'CM VS2010, Debug, x64, Schannel, Shared, Build-tests & examples, XP'
+    - job_name: 'CM VS2010, Debug, x64, Schannel, Shared, Build-tests & examples'
       APPVEYOR_BUILD_WORKER_IMAGE: 'Visual Studio 2013'
       PRJ_GEN: 'Visual Studio 10 2010'
       TARGET: '-A x64'
-      WINTARGET: 0x0501  # Windows XP
       PRJ_CFG: Debug
       SCHANNEL: 'ON'
       SHARED: 'ON'