feat!: Make background of Alert white again

[VincentSmedinga]

Describe the pull request

Thank you for contributing to the project!
Please use this template to help us handle your PR smoothly.

What

1. Reverts feat!: Change colours for Alert #1757 where we added light background colours to Alert.
2. Adds and uses a token to make the background explicitly white.

Why

1. Our branding department doesnt’t (yet?) accept these colours.
2. Because it was transparent, which may have side effects. It didn’t show because our Screen makes the background white, but a component must be fully responsible for its appearance. This also allows other NLDS communities to set a different background colour.

How

Changed tokens and classes.

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• None