Redis 5 has been EOL for a few years, and didn't get updates for the
latest high-severity CVEs. We're already using 6 in most places
(fleetctl preview, recommended reference architectures, managed cloud
environments) so it's safe to set 6 as the new minimum.
Co-authored-by: Noah Talerman <47070608+noahtalerman@users.noreply.github.com>