Drop Windows VISITA from the supported platform

Boost `1.88.0` introduced a feature [^1] that makes use of the Windows API, but it
uses API functions that are only available with `PSAPI_VERSION >= 2` and
Windows VISITA only supports `PSAPI_VERSION == 1`. Actually, that new feature
can also be disabled by setting the `BOOST_STACKTRACE_DISABLE_OFFSET_ADDR_BASE`
macro, but since it seems to be a useful feature and isn't even disabled by default,
we can just drop it that ancient Windows version instead of disabling it.

[^1]: boostorg/stacktrace#200

Author: yhabteab

lib/base/win32.hpp

@@ -5,7 +5,7 @@
 
 #define WIN32_LEAN_AND_MEAN
 #ifndef _WIN32_WINNT
-#define _WIN32_WINNT _WIN32_WINNT_VISTA
+#define _WIN32_WINNT _WIN32_WINNT_WIN7
 #endif /* _WIN32_WINNT */
 #define NOMINMAX
 #include <winsock2.h>