doc: added version 7 to N-API version matrix

Node.js version 14.12.0 released the N-API version 7, but it's not
present on the N-API version matrix. This change  should fix the problem
in the documentation.

PR-URL: https://github.com/nodejs/node/pull/35319


Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>