RELEASES.md: clean up whitespace
RELEASES.md: remove obsolete arguments

As of this writing, release-version.sh now automatically infers certain
defaults based on the GAV of the project being released. This makes it
much simpler to use, and hence much less error-prone. And consequently,
these instructions are now substantially simpler.
RELEASES.md: link to 2.0.0-beta-7 blog post

The beta3 blog post was good, but the beta-7 one is more substantial,
and closer to the format we want for future blog posts.