Use Rails load_defaults for relevant gemfile in test runs

[mjankowski]

All non-EOL'd Rails version at this point support a load_defaults method which can be used to tell Rails which version defaults to start with.

Some of the rails-version-gemfile test runs currently have deprecations about options that are either changing or going away or whatever, but these can generally be silenced by telling a given version to use it's own defaults, which is what this PR does.

Example of warning: https://github.com/ElMassimo/vite_ruby/actions/runs/9424262054/job/25964175006#step:6:9

The one oddity here (explained with inline comment in the diff) is the behavior of media: screen. In versions prior to 7.0, the media attribute in stylesheet tags would default to screen. That was changed in 7.0 to not be there by default, a behvaior which has a config option to enable/disable - rails/rails@1280620

I've chosen here to do the smallest diff approach - which is to just enable the option, which keeps the link method helper in helper_test working as-is (has a default media attribute). Another approach would be to leave the default off, and then modify that helper to default to no media, and only add one if passed in.

[ElMassimo]

Thanks Matt!