api: Like zulip-mobile, don't allow connecting to Zulip Server <7.0

[chrisbobbe]

This goes rather farther than #1456, which is for bumping this number just to 5.0. This is OK; we recently bumped zulip-mobile to use 7.0, in the v27.235 release.

zulip-mobile shows a "nag banner" to nudge server admins to upgrade. As of v27.235 (PR zulip/zulip-mobile#5922), that banner appears on 7.x servers, the latest 7.x having recently aged out of our 18-month support window. So we should feel comfortable nudging this number in zulip-flutter to 8.0 once v27.235 has been out for a little while.

Fixes: #1456

[chrisbobbe]

I added a

// TODO(#268) address all TODO(server-5), TODO(server-6), and TODO(server-7)

even though #268 really only corresponds to TODO(server-5)s. I can update or replace #268 once this is merged, if that's helpful.

[gnprice]

Thanks! Looks good; merging.

I think it'll be cleanest to let #268 remain for just the TODO(server-5)s, and have separate issues for server-6 and server-7. That'll let us separately track fixing one and fixing the next, which might happen in separate PRs.

It's fine for the TODO to refer initially to just #268, though. When we go to handle the TODO(server-5)s, we'll naturally be acting on that line, and we can update it to refer to the successor issues then.