beginDate should be an identity field for `staffSectionAssociation`

staffSectionAssociation doesn't seem to have begin_date as part of its primary key all the way up to 5.3.1. This seems like an oversight to me.
 
Edge case where having begin_date is useful: #

1. Staff member A teaches a section 111 from 2021-8-30 to 2021-10-30 then goes on maternity leave
2. Long term sub B teaches section 111 from 2021-11-01 to 2022-01-31 while A is on maternity leave
3. Staff member A then is reassigned a second time to section 111 from 2022-02-01 to the end of the school year

 
Right now, step 3 would upsert the record from step 1, which doesn't paint a full picture of the staffSectionAssociation history for the current school year. In this case the ODS would only have a single staffSectionAssociation for section 111 and staff member A.