Disallow connecting to server 4.x

[gnprice]

This is a reduced, pre-launch version of:

• Require Zulip Server 5 #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: Disallow connecting to unsupported ancient servers #267
• A fuller version of this issue, to be done post-launch: Require Zulip Server 5 #268

[gnprice]

I was about to go also file an issue for us to have the legacy app start warning people that older than Server 5 isn't supported and they should (get their admins to) upgrade… but happily we already did that back in 2023 (thanks @chrisbobbe!):

• api: Don't allow connecting to servers <4.0; show nag banner on <5.0 zulip-mobile#5762

The changelog says that went out to all users in v27.216 and earlier, dated 2023-10-10.

[gnprice]

This issue isn't yet ripe to carry out (at least not to merge a PR doing it): out of an abundance of caution, I'd like to first get out a beta release containing #267 to disallow servers older than 4.x, and only then merge this change so that it goes out in the next or later beta release.

[chrisbobbe]

the legacy app

I've just sent zulip/zulip-mobile#5920 to nudge more server admins via the legacy app :)