Enable `print_only_version_number` by default

[deivid-rodriguez]

print_only_version_number is a feature flag to enable printing only 4.0.0 when bundle --version is run rather than Bundler version 4.0.0. Seems fine to me and I would've probably not considered it a breaking change at all these days.

[hsbt]

+1. gem -v already printed only version number.