doc: standardize on End-of-Life capitalization

Use "End-of-Life" everywhere and not "End-of-life" or "End-Of-Life".

PR-URL: https://github.com/nodejs/node/pull/26442


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>