chore: Update alert followsDesignSystem to true

What does this MR do?

Updates the popover and modal's followsDesignSystem to true since they have a complete audit and styles updated in GitLab UI:

• gitlab-org/gitlab-services/design.gitlab.com#652 (closed)
• !2094 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

N/A

Accessibility

N/A

Closes #1363 (closed)