doc: add N-API version 6 to table

We missed adding version 6 to the compatibility
table when we defined version 6. Add it along with the
versions that we know will include version 6.

PR-URL: https://github.com/nodejs/node/pull/32829


Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>