CI on jruby 9.4

jruby 9.3 is compatible with ruby 2.6 which is no longer supported

Author: amatsuda

.github/workflows/stable.yml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby: [2.7, '3.0', 3.1, 3.2, ruby-head, jruby-9.3]
+        ruby: [2.7, '3.0', 3.1, 3.2, ruby-head, jruby-9.4]
 
     env:
       BUNDLE_WITHOUT: "benchmark"