Decide which Hijris fall back to other Hijris

[sffc]

Since HijriSimulated is no longer in AnyCalendar, we should make a decision on how exactly the CalendarAlgorithm fallback is performed when mapping from a locale to a Hijri calendar.

We consider this something we can continue to iterate on after 2.0. If a client like Gecko wants us to stabilize the fallback, we can adopt a stability policy post-2.0.

CC @hsivonen

[Manishearth]

Since HijriSimulated is no longer in AnyCalendar, we should make a decision on how exactly the CalendarAlgorithm fallback is performed when mapping from a locale to a Hijri calendar.

... whoops

History: we decided to remove it in #6210, and then we forgot because it wasn't in the right milestone.

We are currently discussing deprecation strategies in #7311 and #7320