Add release notes for dex downgrade
Add release notes for dex downgrade

[#4699](https://github.com/concourse/concourse/pull/4699)

Signed-off-by: Denise Yu <d2c4da7d6ea264fda124d8d8d8780e90b8d0b85a@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse