mgr/zabbix: fix invalid escape sequence

mgr/zabbix: fix invalid escape sequence

Get rid of DepricationWarning which, according to [1],
is going to become a SyntaxError:

  /home/jenkins-build/build/workspace/ceph-pull-requests/src/pybind/mgr/zabbix/module.py:134: DeprecationWarning: invalid escape sequence '\['
    uri = re.match("(?:(?:\[?)([a-z0-9-\.]+|[a-f0-9:\.]+)(?:\]?))(?:((?::))([0-9]{1,5}))?$", server)

1. https://docs.python.org/3.11/library/re.html

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>