doc: update GOVERNANCE.md

This consists of some hopefully-uncontroversial
simplifications/clarifications to the text. The one substantial change
is to update Node.js Board of Directors to be the OpenJS Board of
Directors.

PR-URL: https://github.com/nodejs/node/pull/30259


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>