Describe the pull request

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What

Updates the design of Alert:

• The left border has become thick, and the severity icon has moved into it.
• The header is now required, enabling a simpler layout that allows the description to flow entirely to the right edge.

Additionally, ‘info’ is now the default severity, which has slightly changed the API and removes the need for a second class to be added at all times.

Why

The design has changed to make Alerts more prominent while adhering to the requirements of the current branding.

How

n/a

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

• UX agrees that the title of an Alert can be made required. This simplifies the placement of the close button ; a PR for this will follow.
• A default severity has not defined yet. Omitting its class name (using only ams-alert) renders an Alert with a black outline. UX indicates that the ‘info’ severity should be the default. A PR for this will follow.
• PRs will also follow to remove the incorrect example of the list in an Alert, and new tickets will be added to the backlog to design for this.