chore(deps): Switch from chrono to jiff for datetime support.

[jetuk]

Chrono is being soft-deprecated and most of the ecosystem
moving to jiff.

BREAKING CHANGE: Public DataType removed. Timestepper now uses
Datetime from jiff directly.

[jetuk]

I have realised that by removing DateType this will not round-trip the start / end date quite the same as before. The time component becomes 00::00::00 when not present on deserialisation, but is then explicitly serialised. I.e. the fact it was not present is lost. Not sure if that is problem.

One difference with Chrono is that jiff::civil::Date will successfully parse a date-time string dropping the time component. Therefore the previous approach of trying a date only, and then falling back to date-time will not work. It is either the implementation above, or a custom deserialisation (possibly using jiff's Pieces to work out if a time component was provided. The same issue will be present in Pywr v2 if that is moved to jiff.