[ruby] fix ruby interop build (#35934)

Followup fix for #35911

Closes #35934

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35934 from apolcyn:ruby_stuff fdafeffc05
PiperOrigin-RevId: 607836125

tools/dockerfile/interoptest/grpc_interop_ruby/build_interop.sh

@@ -27,7 +27,7 @@ ${name}')
 cp -r /var/local/jenkins/service_account $HOME || true
 
 cd /var/local/git/grpc
-rvm --default use ruby-2.7
+rvm --default use ruby-3.0
 
 # build Ruby interop client and server
 (cd src/ruby && gem install bundler -v 2.4.22 && bundle && bundle exec rake compile)