Deprecate the "lookup region port" task and make field non-optional.

[jmpesp]

#5955 adds a background task that will poll Crucible agents for the ports that regions were allocated with. This is required because ports were originally not recorded in CRDB, but after that PR lands they will be filled in. At some point in the future, all region records will have an associated port, and new allocations will record the port returned - at that time, deprecate this task, and make the port field non-optional.