Switch off matrixbot

It appears this repository is using https://github.com/matrixbot for some actions.

Without digging deeper, this feels like it is a leftover from the fork-and-AGPL that moved MAS from matrix-org to element-hq. While the element-admin repo is newer, I mean leftover here as still stuck in the heads of the devs or perhaps "out of convenience". ;)

Since plenty time has passed to implement this proactively, I assume that this must have slipped under the radar.

I would like to see Element complete the separation from the Foundation here. Perhaps Element could switch to the already existing https://github.com/elementbot or https://github.com/RiotRobot.