doc: update maintaining openssl guide

Update the maintaining openssl guide to mention following the
instructions in the v14.x-staging version of the guide for the
v14.x-staging branch as the instructions for the quic fork use
a git clone/checkout of the forked repository while the previous
instructions for the non-forked openssl use the release tarball.

Remove references to End-of-Life Node.js 10 and update for Node.js
12 as it is necessary to regenerate the configution files.

PR-URL: https://github.com/nodejs/node/pull/39878


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>