`use_data()`: match R dependency to `version`

[dpprdan]

version = 3 is only supported by R >= 3.5.0, so this should be the minimal dependency version, IMHO.

Are there tests for use_dependency() in use_data()? I haven't found any (but possibly not looked hard enough?).

Should version default to NULL or 3 now or anytime soon? 2 feels outdated for current packages. I guess there is something to be said for a fixed version as default (instead of NULL = default depending on (local) R version), but the downside is that it gets outdated. It's probably better to nudge users into setting the desired version themselves (via use_data_raw()) instead of relying on the default?