Migrate to Ruby 3.0

[skalee]

Ruby 3.0 brings significant performance boost. Migration is expected to be simple, and this gem should greatly benefit from that.

[ronaldtse]

Agree especially since this gem is not used downstream and will not affect others.

[skalee]

This requires changes in some or all Relaton gems, typically because of https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/. I've started experimenting already: relaton/relaton-bib#38, but I presume other Relaton gems need to be fixed too. Also some development dependencies aren't compatible with Ruby 3 yet.

[ronaldtse]

This is resolved in #207