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

docs/examples/block_ip.c

@@ -38,11 +38,7 @@ int main(void)
 }
 #else
 
-#ifdef _WIN32
-#if !defined(_WIN32_WINNT) || _WIN32_WINNT < 0x0600
-#undef _WIN32_WINNT
-#define _WIN32_WINNT 0x0600  /* Requires Windows Vista */
-#endif
+#ifdef _WIN32  /* Requires Windows Vista+ */
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include <windows.h>