Ensure older agents can upgrade to Puppet 7 after changing the default digest algorithm

Changing the default digest_algorithm setting to sha256 in puppet will break remote filebuckets due to an agent-server digest algorithm mismatch (e.g. md5 on server, sha256 on agent).

We need to make sure older agents can cleanly upgrade to Puppet 7. To do this, we probably need to disable remote file buckets in the puppet_agent module.