Mastodon Node: consider adding back `server_version`.

At a minimum, we'll want to document which Mastodon-like instance version is being tested and it would be easier (and less error prone) to ask the instance for that information.

It's not clear to me what versions the pseudo-Mastodon APIs report. I know some client apps require specific versions of Mastodon, so using an arbitrary version number could cause problems in those cases.

Also, by my counts, there are > 300 different Mastodon versions (not including Mastodon-compatible apps) deployed in the Fediverse. It could be interesting to test some of the older versions at some point.