meta: move stefanmb to Collaborator Emeriti list

I asked Stefan if they would mind being moved to Collaborator Emeritus.
They responded that they were not active in Node.js right now and that
moving them was fine.

PR-URL: https://github.com/nodejs/node/pull/27502


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>