Update CI to JRuby 9.4

Author: headius

.github/workflows/ruby.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest ]
-        ruby: [ jruby-9.1.17.0, jruby-9.2.9.0, jruby-9.2.11.0, jruby-head ]
+        ruby: [ jruby-9.4.12.0, jruby-head ]
 
     runs-on: ubuntu-latest