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".
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines