Enable `setup_makes_kernel_gem_public` by default

[deivid-rodriguez]

setup_makes_kernel_gem_public is a feature flag about some improvement regarding the integration with RubyGems that was considered a breaking change at the time. Needs investigation of what it involves exactly.

[deivid-rodriguez]

So the story here is that requiring bundler/setup would make Kernel#gem public and expose the method on every object. This was an unintentional deviation from RubyGems and was corrected but then the fix was reverted because it broke someone's code. See rubygems/bundler#5102.

I think it's fine to make this change without way to opt out, remove the setting, call it a breaking change, and ship it with Bundler 2.7.