cephadm: use importlib.metadata for querying ceph_iscsi's version

cephadm: use importlib.metadata for querying ceph_iscsi's version

use importlib.metadata for querying ceph_iscsi's version and fallback to
pkg_resources. as the former is only available in Python 3.8, while
the latter is deprecated.

Refs https://tracker.ceph.com/issues/66201
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit cbd4bede7f258823462f27cb15d86e13eb52c359)

Conflicts:
	src/cephadm/cephadmlib/daemons/iscsi.py [ this file doesn't
	  exist in quincy, get_version() lives in src/cephadm/cephadm ]