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.

Compare the field declarations in schemas/pl72_run_start.fbs and schemas/6s4t_run_stop.fbs with the required-field syntax shown in schemas/ev44_events.fbs. Check the schema and file-ID compatibility implications before making any changes; done means the required fields are represented consistently and the versioning decision is documented or reflected in the change.

Written by the indexing model from the issue text.