ServerCompatBanner: Bump minSupportedVersion to 2.1.0, as planned.

We haven't gotten negative feedback about the banner from the
smaller pool of users connecting to servers earlier than 2.0.0. The
banner was introduced with that threshold (in PR zulip#4750) to beta
users in v27.164 (2021-06-17) and to all users in v27.165
(2021-06-24).

So, proceed with aligning our threshold with what we declare in the
server stable-release doc, and update the code comment to match.

Author: chrisbobbe

src/common/ServerCompatBanner.js

@@ -14,11 +14,9 @@ import { dismissCompatNotice } from '../session/sessionActions';
 import ZulipTextButton from './ZulipTextButton';
 import { openLinkWithUserPreference } from '../utils/openLink';
 
-// In fact the oldest version we currently support is 2.1.0, per our docs:
-//   https://zulip.readthedocs.io/en/4.2/overview/release-lifecycle.html
-// For now we only show this banner for servers older than 2.0.0, though,
-// in order to phase that in gradually.
-const minSupportedVersion = '2.0.0';
+// The oldest version we currently support. Should match what we say at
+//   https://zulip.readthedocs.io/en/stable/overview/release-lifecycle.html#compatibility-and-upgrading.
+const minSupportedVersion = '2.1.0';
 
 const styles = createStyleSheet({
   wrapper: {