Set the volatility of the timestamptz version of date_bin() back to immutable

Set the volatility of the timestamptz version of date_bin() back to immutable

543f36b43d was too hasty in thinking that the volatility of date_bin()
had to match date_trunc(), since only the latter references
session_timezone.

Bump catversion

Per feedback from Aleksander Alekseev
Backpatch to v14, as the former commit was