Update remaining references to primary/secondary namespaces

[rust-lightning] / pending_changelog / kvstore.txt

## Backwards Compatibility

* Users migrating custom persistence backends from the pre-v0.0.117 `KVStorePersister` interface can use a concatenation of `[{primary_namespace}/[{secondary_namespace}/]]{key}` to recover a `key` compatible with the data model previously assumed by `KVStorePersister::persist`.