Remove jruby/rails-6.1 from matrix

jruby is now 3.4 and that version no longer works
with rails 6.x

furthermore, jruby is not yet compatible with
rails 7.1 and 8.0.

Author: solnic

.github/workflows/sentry_rails_test.yml

@@ -81,8 +81,6 @@ jobs:
             rails_version: "8.0.0"
           - ruby_version: "3.4"
             rails_version: "8.0.0"
-          - ruby_version: "jruby"
-            rails_version: 6.1.0
           - ruby_version: "3.2"
             rails_version: 7.1.0
             options: