tls: rename validateKeyCert in _tls_common.js

This commit renames validateKeyCert to validateKeyCertArg to avoid
confusing this with something that would validate the actual key or
certificate.

PR-URL: https://github.com/nodejs/node/pull/28116


Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>