Fix Float values, Remove 2022-08 forecast value table

[devsjc]

The previous migration created a table as opposed to a partition for forecast_value_2022_08 which wants to either a) be a partition on forecast_value or b) not exist at all. Remove with the next migration.

Also, ForecastValueSQLMixin defines the expected_power_generation_megawatts column to be a float, but the migrations generated by alembic still set the precision to 6. This needs to be investigated as newly added partitions complain about incorrect data types.