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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines