It was originally added by https://github.com/ruby/setup-ruby/pull/253,
in order to fix CI. However, now CI passes without it so I assume this
was fixed by rubyinstaller2.
Force actually bypasses dependency constraints so it's actually
installing the latest Bundler in a version of Ruby that's incompatible
with it.
When a fixed version is passed, it should be the minimum RubyGems
version for the whole matrix, i.e., RubyGems should not be dowgraded
past the default version of the oldest Ruby in the matrix.
David Rodríguez