Update dependencies, drop unsupported ruby versions, create rubocop todo

[zhulik]

Initially I wanted to only update faraday, but found out that almost everything is outdated.

Changes:

• Update faraday minimal version and adjust it's config to using the net_http adapter
• Remove all mentions of ruby < 3 in the Gemfile
• Update rubocop, adjust it's config and generate a todo list. Older versions of rubocop simply do not support newer version of ruby
• Set required_ruby_version to >= 3.0.0
• Remove testing against EoL versions of ruby, run tests against supported ruby versions

Tests and rubocop are green.

TODO:

• Test against prometheus. UPD: it works correctly

If you don't mind and everything is ok with these changes, I'll also open a couple of follow ups to deal with rubocop todos and enable new cops.

What do you think about moving from travis to github workflows? UPD: is travis even working?

[yaacov]

hi, thank you