Simplify ruby installations

Refs #4048

* We don't currently need a specific version suffix on the package names,
  since ubuntu only ships with one ruby version (and newer versions on
  e.g. 23.04 are fine).
* We don't need to explicitly install libruby, since it's pulled in by
  other packages as required.
* Ubuntu again ships a decent enough version of bundler for our needs,
  so we don't need to install it via rubygems.