Revert "Pin JRuby to v9.3.9.0 in CircleCI builds"

This reverts commit b8e6d06.

The problem identified in #591 was reported and investigated in
jruby/jruby#7492, fixed in jruby/jruby#7536 and released in JRuby
v9.4.1.0 [1]. The latest JRuby release is v9.4.2.0, so reverting the
commit which pinned the JRuby version now works.

Fixes #591.

[1]: https://github.com/jruby/jruby/releases/tag/9.4.1.0

.circleci/config.yml

@@ -16,7 +16,7 @@ jobs:
       - run: ruby --version
       - when:
           condition:
-            equal: [ "jruby:9.3.9.0", << parameters.docker-image >>]
+            equal: [ "jruby:latest", << parameters.docker-image >>]
           steps:
             - run: apt-get update
             - run: apt-get install -y git
@@ -82,7 +82,7 @@ workflows:
                 - ruby:3.1
                 - ruby:3.2
                 - ruby:latest
-                - jruby:9.3.9.0
+                - jruby:latest
               gemfile:
                 - Gemfile
       - build: