storage_service: Make heartbeat response log debug level
storage_service: Make heartbeat response log debug level

It is too noisy. It is supposed to be debug level. I forgot to move back
to debug level after testing during development.

Refs #8825

Closes #8960

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla