doc: mark Node.js v15.x as EOL

Today is 2021-06-01, which is the date v15.x release line is marked as
End-of-Life.

Refs: https://github.com/nodejs/Release/blob/main/schedule.json

PR-URL: https://github.com/nodejs/node/pull/38891


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Nitzan Uziely <linkgoron@gmail.com>