Mark the trust kvs backend deprecated, for removal in Kilo

Although the kvs.Base was marked as deprecated, the individual
backends were not.  This marks the kvs trust backend.

Partially-implements: bp deprecated-as-of-juno

Change-Id: I8b9b08ff7eefa6ad00db791c7077ddca5853ac6b
Merge "Mark the trust kvs backend deprecated, for removal in Kilo"