[ci] remove jruby-head until we can address #2490

Author: solnic

.github/workflows/sentry_ruby_test.yml

@@ -59,6 +59,8 @@ jobs:
               rack_version: 3.1,
               redis_rb_version: 5.3
             }
+        exclude:
+          - { ruby_version: 'jruby-head' }
     steps:
       - uses: actions/checkout@v1