Update acme-client to v2.0.9

This brings in Ruby 3.0 support and other minor fixes. This also matches
the version already used by Omnibus
(https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5590).

Diff: https://my.diffend.io/gems/acme-client/2.0.6/2.0.9

Changelog: fixed