doc: remove outdated COLLABORATOR_GUIDE sentence about breaking changes

The TSC has delegated authority over LTS and Current branches to the
Release WG. Remove the bullet point about TSC having authority to
determine that a breaking change is necessary on LTS and Current release
branches. Retaining that authority would require de-chartering the
Release WG.

Fixes: https://github.com/nodejs/TSC/issues/660

PR-URL: https://github.com/nodejs/node/pull/25780


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>