doc: revise breaking changes material in COLLABORATOR_GUIDE

* Remove unnecessary paragraph explaining why Current and LTS have
  stability guarantees that master branch does not. (Leave material
  explaining what those stability guarantees are.)
* Upgrade advisory and passive "Collaborators should take significant
  care" to more direct "Take significant care".

PR-URL: https://github.com/nodejs/node/pull/25730


Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>