TODO: remove base64 dependency after upcoming acme-client release

[geemus]

When we removed sinatra, we started seeing errors against head ruby because the base64 gem was no longer a downstream dependency and it is no longer stdlib in 3.4+.

So, we added the direct dependency to clear the error.

We don't directly even use base64 though, and just inherit the dependency from our usage of acme-client. As such, I made an upstream PR to address the issue there, which has subsequently been merged: unixcharles/acme-client#241

So now, once acme-client has released again we should follow up with removing it here again to simplify and avoid potential dependency version issues moving forward.