github-zephyrproject-rtos: Update contributors with all members

Add all maintainers and collaborators to the contributors.

This ensures that a user is not removed and re-added to the GitHub
organisation during a role change.

Note that the `sync_team_membership.sh` script now handles this
automatically.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio