Made `max` field optional in the moveRange command. Now, the user can specify either: min only (as before), min + max (as before), or max only (new).
In the docs, we will need to change the moveRange command page to note that min is optional if max is specified and vice versa.
And add that if the user only specifies a max key, given a chunk C with the max bound `max`, the min key will be determined as
1. If the data size of the range between min(C) and max is less than the per-collection chunk size or the default chunk size, the chunk's min is selected as min = min(C)
2. otherwise key min > min(C) where min depends on the configured chunk size

Description of Linked Ticket

SERVER-64148 introduced the possibility of invoking `moveRange` by specifying only the `min` bound. Purpose of this ticket is to allow specifying only the `max` bound, resulting in the index getting scanned backwards to find a suitable spit point.