screens: Add notes about using C++20 features

src/displayapp/screens/WatchFaceCasioStyleG7710.cpp

@@ -276,6 +276,7 @@ void WatchFaceCasioStyleG7710::Refresh() {
         std::chrono::system_clock::to_time_t(std::chrono::time_point_cast<std::chrono::system_clock::duration>(currentDateTime.Get()));
       tm* tmTime = std::localtime(&ttTime);
 
+      // TODO: When we start using C++20, use std::chrono::year::is_leap
       int daysInCurrentYear = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0 ? 366 : 365;
       uint16_t daysTillEndOfYearNumber = daysInCurrentYear - dayOfYear;
 

src/displayapp/screens/settings/SettingSetDate.cpp

@@ -28,6 +28,7 @@ namespace {
   int MaximumDayOfMonth(uint8_t month, uint16_t year) {
     switch (month) {
       case 2: {
+        // TODO: When we start using C++20, use std::chrono::year::is_leap
         if ((((year % 4) == 0) && ((year % 100) != 0)) || ((year % 400) == 0)) {
           return 29;
         }