api: Mark GetServerSettingsResult.zulipMergeBase as required

See "Feature level 88" from Zulip API changelog:

   https://zulip.com/api/changelog

This is one of the few places (along with registerQueue) where it's
important that we smoothly handle attempting to connect to an
ancient server where this field might indeed be missing.  Happily,
we do handle those since commit 733bb88 (zulip#1783).

Author: PIG208

lib/api/route/realm.dart

@@ -36,7 +36,7 @@ class GetServerSettingsResult {
 
   final int zulipFeatureLevel;
   final String zulipVersion;
-  final String? zulipMergeBase; // TODO(server-5)
+  final String zulipMergeBase;
 
   final bool pushNotificationsEnabled;
   final bool isIncompatible;