doc,lib,test: rename HKDF 'key' argument

PR-URL: https://github.com/nodejs/node/pull/39474
Refs: https://github.com/nodejs/node/issues/39471


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>