Improve alert component accessibility (colors, icons, etc)

Problem to solve

The new alert component introduced back in https://github.com/gitpod-io/gitpod/pull/8783 was the next iteration of the alert component improved for https://github.com/gitpod-io/gitpod/issues/7613.

🅰️ We're tracking in https://github.com/gitpod-io/gitpod/issues/9411 the pending migration of the previously used InfoBox and AlertBox components to the new Alert component.

🅱️ However, there's still room for improving the alert component design including colors, icons, and variations of those in light and dark themes.

See also relevant discussion (internal).

Alert Component / Info Variant / Light Theme	Alert Component / Info Variant / Dark Theme

Alert Component / Warning Variant / Light Theme	Alert Component / Info Variant / Dark Theme

Proposal

This issue acts as a placeholder for tracking these improvements and the next iteration of the alert component.

Thankfully, now that the new alert component is in place we will only have to update the designs in one place. 🔮

Designs

TBD (To be discussed)

Start with components/dashboard/src/components/Alert.tsx and review the linked discussion and screenshots for the proposed light and dark variants. Designs are still TBD, so first clarify the colors, icons, and variations before changing the component. Done means the agreed accessibility and design improvements are implemented in the Alert component.