[DTOSS-9752] add starts_at to ClinicSlot#829

cameronhargreaves1-nhs wants to merge 1 commit intomainNHSDigital/dtos-manage-breast-screening:mainfrom

DTOSS-9752-clinic-start-time-bugNHSDigital/dtos-manage-breast-screening:DTOSS-9752-clinic-start-time-bugCopy head branch name to clipboard