`cli.search` used deprecated `get_all_items_as_dict`

[TomAugspurger]

Followup to #277.

In

pystac-client/pystac_client/cli.py

Line 30 in caf8dd2

feature_collection = result.get_all_items_as_dict()

cli.search uses the deprecated get_all_items_as_dict.

It should be updated to use the replacement.