core [nfc]: Remove TODO(zulip#1837) for clearing TODO(server-6), which is done!

Fixes zulip#1837.

Author: chrisbobbe

lib/api/core.dart

@@ -14,7 +14,6 @@ import 'exception.dart';
 ///
 /// When updating this, also update [kMinSupportedZulipFeatureLevel]
 /// and the README.
-// TODO(#1837) address all TODO(server-6)
 // TODO(#1838) address all TODO(server-7)
 const kMinSupportedZulipVersion = '7.0';