Require Zulip Server 6

[gnprice]

This is the next round after:

• Require Zulip Server 5 #268

See there for background.

Zulip Server 6 was released 2022-11-17:
https://blog.zulip.com/tag/major-releases/
so it reached 18 months in 2024-05, which means that earlier releases are no longer supported. So the calendar is ripe for us to sweep the codebase for TODO(server-6) comments and resolve them.

Prerequisite issues:

• Disallow connecting to unsupported ancient servers #267
• Require Zulip Server 5 #268