[terra-alert] Deprecate terra-alert

[mjhenkes]

Feature Request

Description

terra-application provides a notification banner. That components is recommended over usage of this component.

• update the repo readme and badge

• update any of the component doc pages, using the notices component. In this case, the maintenance notice variant would be appropriate with similar phrasing as used for terra-application-navigation

• update the component readme (using just straight markdown), update the badge
  adding an independent deprecation guide doc page. This doc should contain what deprecation means, why we've made the change and how consumers should move forward.

After PR

• npm deprecate with a message (we should use the exact same message as what is in our notices component on the main doc pages so these match, plus also on the component readme.md - all can link to our deprecation guide doc).

Audit #3039

Additional Context / Screenshots

@ Mentions

[stale]

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