Remove Windows XP and Vista support

Supporting XP requires too many hacks, and while it wouldn't be very
difficult to support Vista as long as we support Windows 7, it's still
not worth it because nobody uses this system anyhow.

Remove most of XP-specific code and don't use wxDynamicLibrary for using
the functions not present in it any longer.

Don't use <wspiapi.h> neither as we shouldn't use it any more.

Update some comments to not mention Windows versions not supported any
longer and also remove mentions of "Vista and newer Windows versions"
from the documentation as this is now always the case.

This commit is best viewed ignoring whitespace-only changes.

docs/doxygen/mainpages/introduction.h

@@ -79,10 +79,9 @@ paragraphs, but here are some of the benefits:
 wxWidgets first-tier "ports", ie implementations of wxWidgets API, are:
 
 @li wxMSW: This is the native port for Microsoft Windows systems (from Windows
-XP up to Windows 11), either 32 or 64 bits. The primarily supported compilers
-are Microsoft Visual C++ (versions 2005 up to 2022 are supported, at least 2010
-is recommended) and GNU g++ (either from the traditional MinGW, TDM-GCC or
-MinGW-w64 distributions).
+7 up to Windows 11), either 32 or 64 bits. The primarily supported compilers
+are Microsoft Visual C++ (versions 2015 up to 2022 are supported) and GNU g++
+(either MinGW-w64 or TDM-GCC).
 
 @li wxGTK: wxGTK2 and wxGTK3 are the ports using GTK+ library version 2.x and
 3.x respectively. They are very similar, with wxGTK2 being, however, more

docs/doxygen/mainpages/platdetails.h

@@ -81,8 +81,8 @@ In order to configure wxWidgets to compile wxX11 you will need to type:
 
 @section page_port_wxmsw wxMSW
 
-wxMSW is a port of wxWidgets for the Windows platforms (Windows XP and later
-are supported). wxMSW provides native look and feel for each Windows version.
+wxMSW is a port of wxWidgets for the Windows platforms (current version
+requires Windows 7 or later). wxMSW provides native look and feel for each Windows version.
 This port can be compiled with several compilers including Microsoft
 VC++ 2015 or later, MinGW, Cygwin as well as cross-compilation with a
 Linux-hosted MinGW tool chain.

docs/msw/install.md

@@ -1,7 +1,7 @@
 Installing wxWidgets for Windows       {#plat_msw_install}
 --------------------------------
 
-This is wxWidgets for Microsoft Windows (XP, Vista, 7, 8, 10, 11)
+This is wxWidgets for Microsoft Windows 7 or later (up to 11)
 including both 32 bit and 64 bit versions.
 
 [TOC]

docs/readme.txt

@@ -47,7 +47,7 @@ Supported Platforms
 
 This version of wxWidgets supports the following primary platforms:
 
-* Windows XP, Vista, 7, 8, 10 and 11 (32/64 bits).
+* Windows 7, 8, 10 and 11 (32/64 bits).
 * Most Unix variants using the GTK+ toolkit (version 2.6 or newer)
 * macOS (10.10 or newer) using Cocoa (x86-64 or ARM).