ci: temporarilly pin the rbs version for JRuby

[naitoh]

An error occurred while installing rbs (4.0.3), and Bundler cannot continue.

In Gemfile:
  rdoc was resolved to 8.0.0, which depends on
    rbs
Error: The process '/home/runner/.rubies/jruby-10.1.0.0/bin/bundle' failed with exit code 5

• RDoc 8 breaks Jruby due to rbs dependency. rdoc#1746