[ruby/rack] Enable Rubys M:N thread scheduler for Puma (#8678)

With the M:N thread scheduler thread creation and management cost are reduced.

frameworks/Ruby/rack/rack.dockerfile

@@ -1,6 +1,7 @@
 FROM ruby:3.4-rc
 
 ENV RUBY_YJIT_ENABLE=1
+ENV RUBY_MN_THREADS=1
 
 # Use Jemalloc
 RUN apt-get update && \