deps: Update name of Flutter version to reflect new tag

Fixes today's instance of zulip#1138, unbreaking CI.

The output of `flutter --version` at this commit changed because
the tag 3.28.0-0.0.pre was added (to a commit that's already been
in Flutter main since last week).

That was causing `tools/check flutter_version` to fail:
  https://chat.zulip.org/#narrow/channel/516-mobile-dev-help/topic/tools.2Fcheck.20flutter_version.20failure.20in.20CI.3F/near/1999743

Fix pubspec.yaml to use the new name of this version.

pubspec.yaml

@@ -15,7 +15,7 @@ environment:
   # the corresponding recent version of the Dart SDK.
   # Feel free to update these regularly; see README.md for instructions.
   sdk: '>=3.7.0-224.0.dev <4.0.0'
-  flutter: '>=3.27.0-1.0.pre.744'  # 1c1f35711be76d1d2a9f5e3a730747591e64e14a
+  flutter: '>=3.28.0-1.0.pre.35'  # 1c1f35711be76d1d2a9f5e3a730747591e64e14a
 
 # To update dependencies, see instructions in README.md.
 dependencies: