In 2021, Ruby 2.5 and older are EOL.
We can set the default required Ruby version to 2.6.0 to
encourage people to use newer Ruby.
If the command is executed with old Ruby, it falls back to 2.3.0.
It's still possible to create a gem for older Ruby just by changing
two lines of code (one in gemspec and another is in rubocop.yml).