feat: `jekyll.gemspec` and `.rubocop.yml` disagree about the Ruby version

[michaelnordmeyer]

https://github.com/jekyll/jekyll/blob/master/jekyll.gemspec#L32 has Ruby >=2.5.0, but https://github.com/jekyll/jekyll/blob/master/.rubocop.yml#L16 targets Ruby 2.7.

Choosing Ruby 2.7.0 in jekyll.gemspec would mitigate this and also solve #9160.