Wildcard: `<Alert />` migration

Created by: umpox

closes https://github.com/sourcegraph/sourcegraph/issues/27688 closes https://github.com/sourcegraph/sourcegraph/issues/27689

Description

Automated migration of the alert alert-* styles to the <Alert /> WIldcard component. Powered by this codemod: https://github.com/sourcegraph/codemod/pull/72. Will aim to clean up that PR and merge soon!

This PR:

• Fully migrates usage of Bootstrap alert-* classes to the Wildcard <Alert /> component
• Removes global Alert styles

Notes

• Some manual work was required to support usage of <Alert /> in the /shared folder. Particularly around supporting both the variant and className methods of styling. It raises some future issues that we should aim to solve. We'll need to make it as easy as possible for Wildcard components to support unbranded/branded styles as well as unbranded/branded props. See this commit for more detail: https://github.com/sourcegraph/sourcegraph/pull/29862/commits/893d12d1b4a78c05de1280d5ec04e7d772e22480