Show warning banner on deprecated old servers

[gnprice]

Since #267 (plus #1456) we show an error when the user tries to connect to a server too old for the app to understand its API.

We should also add a softer version: a warning when the user is connecting to a server that by policy we already don't support:
https://zulip.readthedocs.io/en/latest/overview/release-lifecycle.html#client-apps
but haven't yet actually removed support for (and taken the resulting opportunity to simplify the app's code).

The zulip-mobile legacy app has a similar pairing of a hard minimum and a soft, higher minimum: see the logic around kMinSupportedVersion, introduced in zulip/zulip-mobile#5633 . Increasing the soft minimum, and publishing a release with that increase, is useful for giving us confidence in subsequently increasing the hard minimum.

The last 6.x release was a little over 22 months ago:
https://blog.zulip.com/tag/release-announcements/
so well past our 18-month support window already, and when we introduce this banner we can have it immediately start warning on versions older than Zulip Server 7.

References

• PR that added analogous feature in zulip-mobile: api: Don't allow connecting to servers <2.0 zulip-mobile#5633
• PR that last updated that zulip-mobile banner — so this issue is effectively the successor to that PR: api: Don't allow connecting to servers <4.0; show nag banner on <5.0 zulip-mobile#5762
• Previous issue for adding the hard minimum: Disallow connecting to unsupported ancient servers #267