add support for Ubuntu releases within standard support (20.04/22.04)

[Matus Kosut]

There is not much info about support for newer Ubuntu releases.

Latest release on https://charmhub.io/nagios still says 18.04, quoting ubuntu blog which indicates that now it is the best time to give some priority to update nagios charm:

> Time to prepare for Ubuntu 18.04 LTS End of Standard Support on 31 May 2023 (https://ubuntu.com/blog/18-04-end-of-standard-support)

EDIT: also adding that Juju 3+ drops support for 18.04

Is there still a plan to support Nagios or should users with Openstack deployments switch to other monitoring tools (e.g. Prometheus)?

[Eric Chen (eric-chen)]

Hi Matus,

It's pity that we don't have any plan to update this charm anymore.
We will switch to new solution "Caonical Observability Stack" (COS) soon.
https://charmhub.io/topics/canonical-observability-stack
In this stack, the major monitoring system is Prometheus+Alert manager+Grafana. There are clear tutorial, please check it.

thanks!