Warn if project version doesn't match server-version in gel.toml

Local instance on 6.5, but another dev has upgraded the project and set 6.7 in gel.toml. If I run `gel migrate` (or anything else which hits that instance) I should get a warning that I need to run `gel project upgrade`.

Start from the `gel migrate` command and other commands that contact the local instance, then inspect how `gel.toml` and the server version are read. Confirm how `gel project upgrade` represents the required upgrade path. Done means a mismatch produces a warning before the command proceeds, while matching versions do not warn.

Written by the indexing model from the issue text.