https://github.com/rubygems/rubygems/commit/ad95a1dfaa

https://github.com/rubygems/rubygems/commit/f530f8686d

If one upgrades the default copy of Bundler through `gem update
--system`, and then reinstalls Ruby without removing the previous copy.
Then the new installation will have a correct default bundler gemspec,
but a higher copy installed in site_dir.

This causes a crash when running Bundler and prints the bug report
template.

This could probably be fixed in Ruby install script, by removing any
previous Bundler default copies, but if the problem is already there, I
think it's best to print a proper user error.

https://github.com/rubygems/rubygems/commit/ada6de765d

We used `Bundler::RubyVersion.system.gem_version` for a long time, but I
changed this to `Gem.ruby_version` at
https://github.com/rubygems/rubygems/commit/94f96439438e. It's unclear why I did that
though since I believe it was unrelated to the fix in there.

Bootboot patches `Bundler::RubyVersion` to customize how Bundler works
with Ruby versions, and that change broke that.

Since it's unclear to me how to achieve what Bootboot is doing with the
current code, and there was no strong reason for the change, let's
restore it for now.

https://github.com/rubygems/rubygems/commit/8ec36c6017

  Pick from https://github.com/rubygems/rubygems/commit/880dd95996c93adc1e032399816931b243c5fe17

  https://github.com/rubygems/rubygems/pull/5960

  Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>