Change school year to 179 days

We currently set it to 180, following the Atlanta Fed's Policy Rules Database, citing what appears to be a ballpark average across states:

https://github.com/PolicyEngine/openfisca-us/blob/1569edcc9939d6f4c593c22262c4b2d1af60a848/openfisca_us/parameters/usda/school_meals/school_days.yaml#L5

[Census assumes 179 days for its SPM calculation](https://www.census.gov/content/dam/Census/library/working-papers/2021/demo/sehsd-wp2021-20.pdf).

We have a stronger interest in aligning to Census data.