fix(monitor install): Fix ubuntu monitor installation prerequisites
fix(monitor install): Fix ubuntu monitor installation prerequisites

Removed deadsnakes repsitory addition in the scylla monitoring installation prerequisites
(an ancient, unnecessary remnant) and replaced the 'python3.6' reference to 'python3',
since we are no longer using python 3.6

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