Bump CHERIoT RTOS version so we can use newest drivers

DO NOT MERGE WITH THIS COMMIT AS IS; THIS POINTS CHERIOT-RTOS TO COMMITS
ONY A LOCAL BRANCH ON MY FORK. WAIT FOR THE CHANGES TO BE PUSHED ONTO
THE OUR LOCAL CHERIOT-RTOS FORK.

This commit bumps the version of CHERIoT RTOS being used to the head of
the updated `sonata` branch on our local RTOS fork, which pulls in the
latest commits and driver updates from upstream `cheriot-rtos` whilst
also still featuring our few remaining local changes yet to be
upstreamed.

Author: AlexJones0