run start/stops: fields that are required are not marked as required

https://github.com/ISISComputingGroup/streaming-data-types/blob/master/schemas/pl72_run_start.fbs and https://github.com/ISISComputingGroup/streaming-data-types/blob/master/schemas/6s4t_run_stop.fbs don't use the `(required)` suffix after typing their fields, on required fields.

See [ev44](https://github.com/ISISComputingGroup/streaming-data-types/blob/master/schemas/ev44_events.fbs#L6) which _does_ use this syntax correctly

technically this is breaking but i don't know if we really want to change the file id and create a new schema or not - probably not.