doc,meta: codify security release commit message

The release commit message for security releases have conventionally
started with the phrase `This is a security release.`. Codify this
as part of the release process so that the distribution indexer can
use this to detect and mark releases as security releases.

Fixes: https://github.com/nodejs/Release/issues/437
Refs: https://github.com/nodejs/node/pull/27612#issuecomment-490698922
Refs: https://github.com/nodejs/nodejs-dist-indexer/pull/9

PR-URL: https://github.com/nodejs/node/pull/27643


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>