chore: Empty state improvements

What does this MR do?

This MR houses a few updates to the Empty State component. Most of which are behind the scenes.

• Removes the logic for centering descriptions and titles due to the spec being removed from pyjamas 6 months ago (gitlab-org/gitlab-services/design.gitlab.com@871ccd38)
• Updates a few of the classes to rely less on GitLab application css
• Cleans up unnessesary computed properties
• Updates the Deprecated buttons to use the new ones.

Other than the update to the buttons, this component will look exactly the same when application CSS is present.

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
  • [-] Customers Portal: mr_url
  • [-] Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.