[ruby/rack] Revert jruby change to MRI (#10353)

JRuby should not run with MRI.

frameworks/Ruby/rack/rack-jruby.dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:4.0-rc
+FROM jruby:10.0
 
 RUN apt-get update -y && apt-get install netbase -y