Revert "Test on JRuby 9.3"

There are test failures on CI that are likely related to the new
version.

Author: janko

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
           - "2.6"
           - "2.7"
           - "3.0"
-          - "jruby-9.3"
+          - "jruby-9.2"
 
     steps:
       - uses: actions/checkout@v2