Use our .ruby-version file in Bundler so it is managed in only 1 place

[corsonknowles]

This provides a single source of truth and simplifies staying current on Ruby.

Background writeup: https://gorails.com/episodes/bundler-ruby-version-file
Original issue: rubygems/rubygems#6742
Implementation: rubygems/rubygems#6876

[robzolkos]

This change will break the Dockerfile as the .ruby-version file isn't copied over when bundler is run. (I think we reverted this change due to this a while back). Rails main has removed the ruby version from the Gemfile completely last I checked (a couple of weeks ago). Anyway you may get some issues raised with Docker no longer working for some peeps.

[corsonknowles]

Thanks @robzolkos This is a widely used and much beloved new feature of Bundler. I was about to say I'll take a look at what is idiosyncratic about our Dockerfile tomorrow, but from the helpful description you provided, I think we can simply do this:

• Keep .ruby_version file in Docker #523