Use truffleruby stable in the CI matrix.

Author: postmodern

.github/workflows/ruby.yml

@@ -14,7 +14,7 @@ jobs:
           - '3.2'
           - '3.3'
           - jruby
-          - truffleruby-head
+          - truffleruby
     name: Ruby ${{ matrix.ruby }}
     steps:
       - uses: actions/checkout@v2