doc: complete n-api version matrix

We have received feedback indicating that the N-API support matrix as
present now can be misinterpreted to mean that old versions of N-API
are no longer supported on newer versions of Node.js. Filling out the
bottom of the matrix should hopefully make it clear that support for
such old versions continues.

PR-URL: https://github.com/nodejs/node/pull/32304


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>