Disallow connecting to server 4.x

[gnprice]

This is a reduced, pre-launch version of:

• #268

The Zulip Server 5 release went out over 3 years ago, so the 4.x releases are all long out of our announced 18-month compatibility window.

As #268 describes, there are already a few places where we rely on the API of Zulip Server 5 and later. If someone tries using the app on a server older than that, then instead of quietly not working correctly it'd be better to give a clear error. Now that #267 is complete, it'll be straightforward to do that.

(The difference between this issue and #268 is that the latter calls for taking advantage of the new minimum version in order to clear out all the TODO(server-5) items in our codebase. This issue is purely for updating the minimum version set by #267, so as to give the user an error when they're on such an old version.)

References

• This issue is a followup to: #267
• A fuller version of this issue, to be done post-launch: #268