Move myself to the list of Past Contributors

Regarding the board that's the best way I've come up with to record my
membership there. An alternative would have been to create a list of
past legislative periods, but I can imagine that to become quite large
in time. Still, I'm open to change this according to others' opinions.

Once this is merged, I'll:
* leave all GitLab groups
* leave all Mattermost channels except the Prototype Fund/mentoring related ones
* remove my access to all Briar infrastructure (that's only desktop.briarproject.org)
* remove my access to YouTube and PeerTube
* remove my access to Transifex

Regarding Transifex, some translators might finish their translations
for my talk in the next months and I hope that @akwizgran or @grote (the
only other ones with access to YouTube) will upload the subtitles there.
Thank you!

MR as per
https://chat.briarproject.org/briar/pl/6qa5wndxzpdcxc8rcd568keqwy.