Migrate using-packages off of css_colors

[stuartmorgan-g]

Page URL

https://docs.flutter.dev/packages-and-plugins/using-packages

Page source

https://github.com/flutter/website/blob/main/src/content/packages-and-plugins/using-packages.md

Describe the problem

The package used in the first example, css_colors, has been discontinued. We should either

• replace it with a different package (animations maybe?), or
• eliminate that example

The second might be the better approach. The fact that the second example is a plugin doesn't prevent it from showing everything that the first example is demonstrating, and a lot of it is just duplicated. We could use url_launcher as the sole example, and just have the part about it being a plugin and what that means be a note at the end.

Expected fix

No response

Additional context

No response

I would like to fix this problem.

• [ ] I will try and fix this problem on docs.flutter.dev.