doc updates: prerequisite section

As reported here #426 (comment 169318) by @marcin:

Prerequisites section of the Stork ARM is outdated. A couple of examples:

Stork Server and Stork Agent have been tested thoroughly on the Ubuntu 18.04 system. They have been tested and run on the Fedora 31 system as well.

So we fixed list of systems in one place but left this. I think we generally have too many places where we list systems on which it has been tested.

Another place:

The status-get command was introduced in Kea 1.7.3. At this time, Stork works with Kea version 1.7.3 and later versions only, although we intend to backport the status-get command to Kea 1.6.3.

In fact, we have already back ported it to 1.6.3.