api: Make zulipMergeBase required, relying on server 5+, FL 88+

See "Feature level 88" from Zulip API changelog:

   https://zulip.com/api/changelog

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/initial_snapshot.dart

@@ -18,7 +18,7 @@ class InitialSnapshot {
   final int lastEventId;
   final int zulipFeatureLevel;
   final String zulipVersion;
-  final String? zulipMergeBase; // TODO(server-5)
+  final String zulipMergeBase;
 
   final List<String> alertWords;
 

lib/api/model/initial_snapshot.g.dart

@@ -34,7 +34,7 @@ class GetServerSettingsResult {
 
   final int zulipFeatureLevel;
   final String zulipVersion;
-  final String? zulipMergeBase; // TODO(server-5)
+  final String zulipMergeBase;
 
   final bool pushNotificationsEnabled;
   final bool isIncompatible;