Upgrade JRuby to 9.2 within testing matrix

Unfortunately Bundler 2 and JRuby 9.1 are not compatible on GitHub
actions (see here: ruby/setup-ruby#108) which
leads to failures.

JRuby matches (MRI) Ruby 2.5 (which we just removed from the testing
matrix) in terms of features so we may want to even upgrade it

Author: hibachrach

.github/workflows/ruby.yml

@@ -15,7 +15,7 @@ jobs:
         ruby:
           - 2.7.0
           - 2.6.5
-          - jruby-9.1.17.0
+          - jruby-9.2.21.0
         gemfile:
           # These have webpacker:
           - rails_5.2_no_sprockets