Drop end-of-life Ruby 2.5 from testing matrix

This was EOLed over a year ago and doing a `bundle install` with the
current set of packages gives

> minitest-5.16.3 requires ruby version < 4.0, >= 2.6, which is
> incompatible with the current version

Author: hibachrach

.github/workflows/ruby.yml

@@ -15,7 +15,6 @@ jobs:
         ruby:
           - 2.7.0
           - 2.6.5
-          - 2.5.7
           - jruby-9.1.17.0
         gemfile:
           # These have webpacker: