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.
Since rails 7, the `rails yarn:install` command no longer uses
bin/yarn and so a) doesn't detect yarn binaries named "yarnpkg"
(e.g. Ubuntu) and b) doesn't allow us to pass any flags to yarn,
(e.g. "--ignore-engines").
Since we want to avoid making the installation more difficult (by
e.g. installing yarn from other sources, or requiring particular
node versions) we can instead just use our existing bin/yarn script.
The reason Dockerfile did its own thing was to avoid pulling in
big chunks of rails, but bin/yarn is a standalone ruby script.
Fixes#3579
