Update README to respect GitHub site theme (#105189)

* Update README to respect GitHub site theme

GitHub recently added support for specifying an alternate appearance when the site is using a dark theme [(see the changelog)](https://github.blog/changelog/2022-05-19-specify-theme-context-for-images-in-markdown-beta/). I've implemented this here with download links from https://flutter.dev/brand.

* Change alt text

README.md

-# [![Flutter logo][]][flutter.dev]
+<a href="https://flutter.dev/">
+  <h1 align="center">
+    <picture>
+      <source media="(prefers-color-scheme: dark)" srcset="https://storage.googleapis.com/cms-storage-bucket/6e19fee6b47b36ca613f.png">
+      <img alt="Flutter" src="https://storage.googleapis.com/cms-storage-bucket/c823e53b3a1a7b0d36a9.png">
+    </picture>
+  </h1>
+</a>
 
 [![Build Status - Cirrus][]][Build status]
 [![Discord badge][]][Discord instructions]
@@ -85,7 +92,6 @@ Flutter is a fully open-source project, and we welcome contributions.
 Information on how to get started can be found in our
 [contributor guide](CONTRIBUTING.md).
 
-[Flutter logo]: https://github.com/flutter/website/blob/archived-master/src/_assets/image/flutter-lockup-bg.jpg?raw=true
 [flutter.dev]: https://flutter.dev
 [Build Status - Cirrus]: https://api.cirrus-ci.com/github/flutter/flutter.svg
 [Build status]: https://cirrus-ci.com/github/flutter/flutter/master