TODO api: Add TODO to modernize GetServerSettingsResult.zulipMergeBase

Marked as TODO as this might change in the future.
See zulip#904 (comment).

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/route/realm.dart

@@ -36,7 +36,8 @@ class GetServerSettingsResult {
 
   final int zulipFeatureLevel;
   final String zulipVersion;
-  final String? zulipMergeBase; // TODO(server-5)
+  // TODO(server-5): Modernize this once we get to #267.
+  final String? zulipMergeBase;
 
   final bool pushNotificationsEnabled;
   final bool isIncompatible;