doc: clarify weak keys text

Describe small key sizes as "too weak for safe use" rather than
"considered to be too weak for safe use".

PR-URL: https://github.com/nodejs/node/pull/28090


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>