Enable Material 3 in experimental/web_dashboard (#1954)

Enabled Material 3.

Replaced the `ElevatedButton` by `FilledButton` for better contrast.

#### Before Material 3

<img width="1392" alt="Screenshot 2023-07-21 at 15 26 44"
src="https://github.com/flutter/samples/assets/2494376/dea484eb-cb0f-46e1-a6b0-bdfef7bab519">

#### With Material 3

<img width="1392" alt="Screenshot 2023-07-21 at 15 28 22"
src="https://github.com/flutter/samples/assets/2494376/a2a449c1-3a73-4bc0-a270-33b6b9d91fa3">


## Pre-launch Checklist

- [ ] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [ ] I signed the [CLA].
- [ ] I read the [Contributors Guide].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

experimental/web_dashboard/lib/src/pages/sign_in.dart

@@ -88,7 +88,7 @@ class _SignInButtonState extends State<SignInButton> {
           _showError();
         }
 
-        return ElevatedButton(
+        return FilledButton(
           child: const Text('Sign In with Google'),
           onPressed: () => _signIn(),
         );