[System.Core] Fixed wrong DST of TimeZoneInfo with floating date rules.

Fixes #26008
We do not want (transition.DayOfWeek - first) % 7 to be negative.
As (transition.DayOfWeek - first) cannot be lower than -7, adding 7 to it solves the issue.

mcs/class/System.Core/System/TimeZoneInfo.cs

@@ -1086,7 +1086,7 @@ private static DateTime TransitionPoint (TransitionTime transition, int year)
 				return new DateTime (year, transition.Month, transition.Day) + transition.TimeOfDay.TimeOfDay;
 
 			DayOfWeek first = (new DateTime (year, transition.Month, 1)).DayOfWeek;
-			int day = 1 + (transition.Week - 1) * 7 + (transition.DayOfWeek - first) % 7;
+			int day = 1 + (transition.Week - 1) * 7 + (transition.DayOfWeek - first + 7) % 7;
 			if (day >  DateTime.DaysInMonth (year, transition.Month))
 				day -= 7;
 			if (day < 1)