doc: edit reason-for-deprecation text

It's not clear (to me, at least) how describing an API as  "considered
to be unsafe" differs from describing it as simply "unsafe". Use the
shorter, latter wording.

PR-URL: https://github.com/nodejs/node/pull/28098


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>