doc: edit LTS material in Collaborator Guide

* Rewrite some material for simplicity and directness.
* Remove outdated LTS material in the Collaborator Guide. (For example,
  refers to LTS WG for things that are now handled by the Release WG.)
* Minor change to text about force pushing (s/minimize/reduce/).

PR-URL: https://github.com/nodejs/node/pull/26845


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>