Migrate to shared alert component

What does this MR do and why?

Migrates the .alert class to use the shared alert component from GitLab UI.

Screenshots or screen recordings

before	after

How to set up and validate locally

This is a bit difficult to replicate, but returning true in the showLockedWarning computed prop is a workaround for displaying the error.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #353430 (closed)