Add newest ruby versions for CI. And cutoff out of official support ruby versions.

[nekomaho]

Hi!

This gem does not seem to support yet a newer version of rubies. This PR adds settings to CI to test the new versions of ruby.

And I remove old ruby versions that are already excluded from official support.

see also: https://www.ruby-lang.org/en/downloads/branches/

About disabled rules

By removing the old ruby version, the following rubocop rules now detect existing source code. As for these, in order to deviate from the purpose of PR, I add to the setting of exclusion for rubocop.

• Style/FrozenStringLiteralComment
• Style/SafeNavigation
• Style/RedundantFreeze

If you want to enable those settings, I will make another PR.

thanks.

[sds]

Thanks @nekomaho!