improvement(db-events): reduce severity to warning for cql_server connection processing error
improvement(db-events): reduce severity to warning for cql_server connection processing error

Reduce severity to warning for the error message:
'cql_server - exception while processing connection: std::system_error'

scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests