meta: move to emeritus automatically after 18 months

Similar to the rules for automatic removal from active TSC membership,
provide criteria for automatic assignment to emeritus status for
inactive collaborators.

PR-URL: #41155
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Stewart X Addison <sxa@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Gus Caplan <me@gus.host>

GOVERNANCE.md

@@ -67,6 +67,10 @@ See:
 The TSC can remove inactive collaborators or provide them with _emeritus_
 status. Emeriti may request that the TSC restore them to active status.
 
+A collaborator is automatically made emeritus (and removed from active
+collaborator status) if it has been more than 18 months since the collaborator
+has authored or approved a commit that has landed.
+
 ## Technical Steering Committee
 
 A subset of the collaborators forms the Technical Steering Committee (TSC).