rails_edge on 2.7 needs newer RubyGems

[ericproulx]

While working #2292, I found that error

Because every version of rails depends on RubyGems >= 3.3.13
    and rails_edge.gemfile depends on rails >= 0,
    RubyGems >= 3.3.13 is required.
  So, because current RubyGems version is = 3.1.6,
    version solving has failed.

Easy to fix. We add rubygems: latest to the ruby/setup-ruby@v1 step. See https://github.com/ruby/setup-ruby/blob/master/action.yml