Only display downgraded message if on free plan

We've become aware that sometimes Zuora => Customers => GitLab fall out
of sync. It certain contexts a user will receive a false message stating
their account has been downgraded when in fact we haven't downgraded
them.

This commit ensures that the user is on the Free plan, before displaying
the downgraded message.

part of: https://gitlab.com/gitlab-org/growth/engineering/-/issues/5405