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/DEPRECATE.md

@@ -12,13 +12,6 @@ email the
 as soon as possible and explain to us why this is a problem for you and
 how your use case cannot be satisfied properly using a workaround.
 
-## Windows XP
-
-In January 2026, curl drops support for Windows XP and Server 2003. Their
-"mainstream support" ended in 2014, with final updates on May 14, 2019.
-
-Making the new minimum target Windows version Vista / Server 2008.
-
 ## c-ares 1.16.0
 
 In March 2026, we drop support for all c-ares versions before 1.16.0.
@@ -77,3 +70,4 @@ CMake 3.18 was released on 2020-07-15.
  - Windows CE (removed in 8.18.0)
  - Support for Visual Studio 2008 (removed in 8.18.0)
  - OpenSSL 1.1.1 and older (removed in 8.18.0)
+ - Support for Windows XP (removed in 8.19.0)