Drop mongodb server 3.6 support

[hasezoey]

The mongodb nodejs driver will soon be dropping mongodb server version 3.6 (as noted in mongodb nodejs driver 6.9), this means MMS will also drop support for mongodb server version 3.6 (in a major version).

Note that binary version 3.6 is already EOL for almost 3 years and mongodb only commits to supporting a EOL server version in the drivers only for 3 years after EOL.

Note that binary version 3.6 is currently not actually tested in CI and also not available for ubuntu versions above 18.04.

This change will entail that MMS will not be able to download or manage 3.6 binaries anymore.