Execute tests against TruffleRuby 21

Tests are failing in TruffleRuby 22

Author: cjlarose

.github/workflows/tests.yml

@@ -22,7 +22,7 @@ jobs:
           - 2.6
           - 2.7
           - jruby
-          - truffleruby
+          - truffleruby-21
     steps:
       - name: Checkout
         uses: actions/checkout@master