Add Puppet Server 8.x JRuby to unit tests

This commit adds JRuby 9.4.7.0 to unit tests, which is the same version
of JRuby that Puppet Server 8.x uses. (See:
puppetlabs/jruby-utils@1180711.)

.github/workflows/unit_tests.yaml

@@ -23,6 +23,7 @@ jobs:
           - '3.0'
           - '3.2'
           - 'jruby-9.3.14.0'
+          - 'jruby-9.4.7.0'
     runs-on: ubuntu-20.04
     steps:
       - name: Checkout current PR