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.