Rename 'common.VersionInfo' struct to 'common.Info' struct

The following discussion from !5802 (merged) should be addressed:

• @dbickford started a discussion: (+1 comment)

  @tmaczukin : after working with @pedropombeiro on the Rails side, it appeared to make sense to embed the Labels in the VersionInfo field of the payload, which is simply named info in the payload itself. It seems we may want to think about renaming the VersionInfo struct as it has expanded over time to use all general information about the runner 🙂

The JSON API generated from this struct is already called info. And there is no external usage of it in Go, so it should be quite straightforward to rename it.