doc: standardize on _backward_

We use _backward incompatible_ and _backwards incompatible_ with no
discernible pattern in the docs. Follow Chicago Manual of Style and also
our standardization on US English and favor _backward_.

PR-URL: https://github.com/nodejs/node/pull/35243


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>