[Update] Nagios 3 to Nagios 4

[s-makin]

We have two pages that refer to the Nagios 3 package, but this was replaced by Nagios 4 in Bionic (18.04). Nagios 4 is still supported, so it would be great to bring this page up to date.

Pages affected:

• How to use Nagios with Munin
• How to install and configure Nagios Core 3

What's needed:

• Change all mentions of Nagios 3 to Nagios 4
• Test the steps to see if they still work
• Either highlight any steps that don't work anymore when you submit your PR so that we can help fix them, or update them if you are able

[cpaelzer]

As discussed with the great @s-makin - we could remove from the serverguide

• https://documentation.ubuntu.com/server/how-to/observability/install-nagios/
• https://documentation.ubuntu.com/server/how-to/observability/use-nagios-with-munin/

But keep their discourse content as community content, together with the new nice community content of

• https://discourse.ubuntu.com/t/nagios-core-4-installation-and-configuration/51519/1

This https://documentation.ubuntu.com/server/how-to/observability/set-up-your-lma-stack/ or the overview page of observability can refer to what we have left in our docs AND to those community elements if people want to use the older.