Drop support for EOL Ruby versions

Currently, this project supports Ruby back to 2.3.x. I propose that moving forward that we only support major versions of Ruby that are still getting active support by the Ruby core team. These are the major versions that are NOT in the 'eol' status on the Ruby Maintenance Branches page.

That would mean the dropping support for versions of Ruby prior to Ruby 2.6 (released 2018-12-25) and then Ruby 2.7 when Ruby 2.6 is EOL on 2022-03-31.

When dropping support for a Ruby version, the major version of this gem should be bumped (1.x.x --> 2.x.x). Users of the gem who want to continue using an eol version of Ruby should continue using the last major version of the git gem that supported their version of Ruby.

The following changes will need to be made:

• Bump the major version of the git gem.
• The CI build should build with every supported, released major version of MRI Ruby AND the head version of MRI Ruby.
• required_ruby_version in the gemspec will need to be updated.
• TargetRubyVersion in the Rubocop config will need to be updated.