Upgrade JRuby to 9.3 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.

Additionally, due to the `webdrivers` gem as a testing dependency
needing upgrading, 9.2 is also a no-go.

JRuby 9.3 matches (MRI) Ruby 2.6 which we just made the floor of the testing
matrix, so this is likely a good version to match anyhow.

Author: hibachrach

.github/workflows/ruby.yml

@@ -15,7 +15,7 @@ jobs:
         ruby:
           - 2.7.0
           - 2.6.5
-          - jruby-9.1.17.0
+          - jruby-9.3.7.0
         gemfile:
           # These have webpacker:
           - rails_5.2_no_sprockets