doc: remove ES6/ECMAScript 2015 from buffer.md

As the introduction of ES6 features recedes further into the past, it
is less and less relevant (and more and more distracting) to cite it in
documentation text. Remove mention in buffer.md.

Backport-PR-URL: https://github.com/nodejs/node/pull/21590
PR-URL: https://github.com/nodejs/node/pull/19685


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>