Allow the use of Bundler 2.0

[jules2689]

Summary

In #8234 we introduced a change that placed an upper bound on Bundler at 2.0

Bundler 2.0 is on the near horizon with a release expected later this year.

Previous reports claimed that there would be major changes, and while there are breaking changes (like the Gemfile.lock being totally rewritten), it does not block a Rails app from running.

Using Bundler master, I was able to enable Bundler 2.0 with all major deprecations, bundle install and require 'bundler/setup' via bin/rake environment without any issue whatsoever.

In preparation for Bundler 2.0 release, this PR removes that upper bound.

It is important to note that Bundler currently will print a warning but continue to run if you use Bundler 2.0 - so this isn't changing much except that a warning won't be printed.

cc @rafaelfranca

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