MDEV-22615 system_time_zone may be incorrectly reported as "unknown".

TIME_ZONE_ID_UNKNOWN return code from GetDynamicTimeZoneInformation()
does not mean failure.

It only means, daylight saving dates in the returned strct are not valid.
TIME_ZONE_ID_INVALID means failure, in this case  "unknown" should be returned

sql/mysqld.cc

@@ -4176,7 +4176,7 @@ static void get_win_tzname(char* buf, size_t size)
     {0,0}
   };
   DYNAMIC_TIME_ZONE_INFORMATION  tzinfo;
-  if (GetDynamicTimeZoneInformation(&tzinfo) == TIME_ZONE_ID_UNKNOWN)
+  if (GetDynamicTimeZoneInformation(&tzinfo) == TIME_ZONE_ID_INVALID)
   {
     strncpy(buf, "unknown", size);
     return;