Alignment: Alert

The alert component should include the following properties to align with the Design system:

- **children** - `string`
- **color** - `danger`, `info`, `warning`, `success`
- **size** - `sm`, `md`, `lg`

According to our docs, we currently have `severity` in place of `data-color`. This should be renamed to the same property name as the design system uses.

Component documentation from the Design System can be found [here](https://storybook.designsystemet.no/?path=/docs/komponenter-alert--docs&args=data-color:brand3;data-size:sm;open:!false)

Start by locating the alert component and its current documentation, then compare its props with the linked Design System Alert documentation. Done means supporting children, color, and size with the documented values and replacing the current severity property with the design system's data-color name.

Written by the indexing model from the issue text.