Fix: Using MIN_YEAR for a date is probably wrong, Use MIN_DATE instead. (#11444)

Even with strong types ...

src/table/roadtypes.h

@@ -82,7 +82,7 @@ static const RoadTypeInfo _original_roadtypes[] = {
 		0x01,
 
 		/* introduction date */
-		static_cast<int32_t>(CalendarTime::MIN_YEAR),
+		CalendarTime::MIN_DATE,
 
 		/* roadtypes required for this to be introduced */
 		ROADTYPES_NONE,

src/timer/timer_game_calendar.h

@@ -178,6 +178,9 @@ public:
 	/** The date of the first day of the original base year. */
 	static constexpr TimerGameCalendar::Date DAYS_TILL_ORIGINAL_BASE_YEAR = TimerGameCalendar::DateAtStartOfYear(ORIGINAL_BASE_YEAR);
 
+	/** The absolute minimum date. */
+	static constexpr TimerGameCalendar::Date MIN_DATE = 0;
+
 	/** The date of the last day of the max year. */
 	static constexpr TimerGameCalendar::Date MAX_DATE = TimerGameCalendar::DateAtStartOfYear(CalendarTime::MAX_YEAR + 1) - 1;