doc: update crypto.createSecretKey history

Passing strings to `crypto.createSecretKey` was not added until v15.0.0
(specifically with commit dae283d9). The existing documentation implies
its availability in prior versions which may cause confusion.

PR-URL: https://github.com/nodejs/node/pull/35874


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>