This fixes a minor UI bug where when after validating an account,
toggling the shared runners on and off again will cause the validation
error to display again. Users can workaround this by reloading the page.

To avoid this problem, once we've successfully validated, we store that
state and use it to determine to show the warning. This also simplifies
the code.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/334322