RELEASE.md

* drop the detached gpg signature as this doesn't seem required anymore by
anything; the signed git tag is sufficient to auth the released source

* exclude vagrant data from git clean, otherwise this destroys the test
environment which is useful during the release process; this does not
cause any vagrant artifacts to become part of the release source

* drop attaching release artifacts to milestones, as GitLab Releases
have made this practice deprecated

* add diffoscope command for easier tarball comparison