doc: move Shigeki to TSC Emeritus

Shigeki stepped down from TSC in a private email. He intends to stay on
as a Collaborator and to continue working on OpenSSL integration,
crypto, etc. (Since I know people will ask: His message indicates that
this is about having sufficient time to participate. There is no reason
he couldn't be nominated for TSC again in the future if he wished and
had sufficient availability.)

PR-URL: https://github.com/nodejs/node/pull/16195


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>