Add net-protocol BSD-2 license

As announced in https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/, net-http and net-protocol have been promoted from the Ruby stdlib to a default gem. The update to mail_room in gitlab!56913 (merged) will depend on a newer version of net-imap and net-http, which in turn depends on this newly-released net-protocol gem.

These default gems are all released under the Simplified BSD license, so we should be able to use them.