revert removing Twitter, retain BlueSky (flutter#161803)

https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md?plain=1#L11-L13

> The Flutter project expects Flutter's contributors to act
professionally
and respectfully. Flutter contributors are expected to maintain the
**safety
and dignity of Flutter's social environments** (such as GitHub and
Discord).

The Flutter community on X is significant and important. We are the
interface between Flutter and the larger tech community. We have been
disenfranchised by this PR.
flutter#161778

> * Respect people, their identities, their culture, and their work.
> * Be kind. Be courteous. Be welcoming.
> * Listen. Consider and acknowledge people's points before responding.

This change is not respectful, kind, courteous, or welcoming. There
wasn't an opportunity to listen to or consider the affected people.

> Should you experience anything that makes you feel unwelcome in
Flutter's
community, please contact
[[email protected]](mailto:[email protected])
or, if you prefer, directly contact someone on the project, for instance
[Hixie](mailto:[email protected]).

I feel unwelcome.

README.md

@@ -9,6 +9,7 @@
 
 [![Flutter CI Status](https://flutter-dashboard.appspot.com/api/public/build-status-badge?repo=flutter)](https://flutter-dashboard.appspot.com/#/build?repo=flutter)
 [![Discord badge][]][Discord instructions]
+[![Twitter handle][]][Twitter badge]
 [![BlueSky badge][]][BlueSky handle]
 [![codecov](https://codecov.io/gh/flutter/flutter/branch/master/graph/badge.svg?token=11yDrJU2M2)](https://codecov.io/gh/flutter/flutter)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/5631/badge)](https://bestpractices.coreinfrastructure.org/projects/5631)
@@ -101,6 +102,8 @@ Information on how to get started can be found in our
 [flutter.dev]: https://flutter.dev
 [Discord instructions]: ./docs/contributing/Chat.md
 [Discord badge]: https://img.shields.io/discord/608014603317936148?logo=discord
+[Twitter handle]: https://img.shields.io/twitter/follow/flutterdev.svg?style=social&label=Follow
+[Twitter badge]: https://twitter.com/intent/follow?screen_name=flutterdev
 [BlueSky badge]: https://img.shields.io/badge/Bluesky-0285FF?logo=bluesky&logoColor=fff&label=Follow%20me%20on&color=0285FF
 [BlueSky handle]: https://bsky.app/profile/flutter.dev
 [layered architecture]: https://docs.flutter.dev/resources/inside-flutter