doc: revise deprecation semverness info in Collaborator Guide

Simplify and clarify deprecation semverness information in the
Collaborator Guide. Unlike some of the other changes I've made lately,
this one is not merely cosmetic. It changes information about how to
handle deprecations vis-a-vis SemVer. The revised conventions take
advange of `notable change` labels etc. instead of suggesting that
doc-deprecations be treated as `semver-minor`. The idea that a
deprecation is a new feature seems incorrect from a SemVer perspective,
but probably made sense at the time the text was written if we weren't
yet using `notable change` etc.

PR-URL: https://github.com/nodejs/node/pull/26232


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>