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.

doc/guides/maintaining-openssl.md

@@ -7,14 +7,13 @@ currently need to generate three PRs as follows:
 
 * a PR for master which is generated following the instructions
   below.
-* a PR for 14.x following the instruction below based on the
-  14,x branch. This PR should cherry pick back to the active release
-  lines except for the 10.x line.
+* a PR for 14.x following the instructions in the v14.x-staging version
+  of this guide.
 * a PR which uses the same commit from the second PR to apply the
   updates to the openssl source code, with a new commit generated
-  by following steps 2 onwards on the 10.x line. This is
-  necessary because differences in 10.x requires that the
-  configuration files be regenerated specifically for 10.x.
+  by following steps 2 onwards on the 12.x line. This is
+  necessary because the configuration files have embedded timestamps
+  which lead to merge conflicts if cherry-picked from the second PR.
 
 ## Use of the quictls/openssl fork