doc: remove instructions for unsupported Node.js versions

Remove special V8 maintenance instructions for versions of Node.js prior
to 9.0.0. Perform other minor edits in nearby text.

PR-URL: https://github.com/nodejs/node/pull/39185


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>