Use RtlGetVersion() to retrieve the real OS version in wxMSW

Unlike GetVersionEx(), this function still returns the real version and not
the fiction concocted by the OS for each program depending on its manifest.

Also use OSVERSIONINFOEXW instead of OSVERSIONINFOEX as RtlGetVersion() only
exists in the Unicode version.

Closes #15321.

docs/changes.txt

@@ -181,6 +181,7 @@ wxMSW:
 - Correct wxGetOsDescription() for Windows 10 (Tobias Taschner).
 - Make wxListCtrl &c appearance more native on modern systems (Tobias Taschner).
 - Don't send wxActivateEvent for minimized windows (bzcdr).
+- Return correct OS version under Windows 8 and later.
 
 wxOSX/Cocoa: