GNOME Calendar needs to deal with two different
weekday ranges:
1. Sunday → 0 ... Saturday → 6
2. Monday → 1 ... Sunday → 7 (used by GDateTime)
When converting (2) into (1), the correct calculation
is `weekday % 7`.
However, gcal_date_time_get_start_of_week() was doing
`weekday - 1` instead.
Fix it and do the correct calculation instead.
https://gitlab.gnome.org/GNOME/gnome-calendar/issues/451