CI: add JRuby 9.4 to the test matrix

Author: orien

.github/workflows/test.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby_version: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3"]
+        ruby_version: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "jruby-9.4"]
         experimental: [false]
         include:
           - ruby_version: "ruby-head"