ifdef-out WinNT 6.0+ APIs (#10198)

Occasionally I need to build a binary compatible with WinNT 5.1, e.g.
when testing out Wine or ReactOS. There are just a few minor changes
required to have a working build, but as they are not part of the
repository, I always end up doing them from scratch. I would like to
upstream them instead so I don't have to recreate them each time, even
if this is not the most common use case.

src/openrct2/core/String.cpp

@@ -706,6 +706,7 @@ namespace String
     std::string ToUpper(const std::string_view& src)
     {
 #ifdef _WIN32
+#    if _WIN32_WINNT >= 0x0600
         auto srcW = ToWideChar(src);
 
         // Measure how long the destination needs to be
@@ -731,6 +732,10 @@ namespace String
         {
             return String::ToUtf8(dstW);
         }
+#    else
+        log_warning("String::ToUpper not supported");
+        return std::string(src);
+#    endif
 #else
         icu::UnicodeString str = icu::UnicodeString::fromUTF8(std::string(src));
         str.toUpper();