Revise the `MessageNotification` component (#24287)

This PR makes adding icons to the primary and secondary actions, in the
`MessageNotification` component, optional. Also took the opportunity to
remove a probably unnecessary "third action" from it; streamlining the
component API (we had added that for a design that we're not using
anymore). I did keep the "more info" possibility, which may be useful in
the future, though.

Release Notes:

- N/A