[breaking] Fix Date64 semantics

[chmp]

Use Timestamp as the default type for detected date time objects

Removes a lot of the strategy logic and simplifies the overall mapping

• Use Timestamp(..., Some("UTC")) for UTC times
• Use Timestamp(..., None) for naive date times
• Use Date64 for naive dates
• Remove the strategy logic
• Note: Date64 is incorrectly used to represent timestamps, it should actually be date and count the number of days ("values of Date64 are treated as the number of days, in milliseconds, since the UNIX epoch. Therefore, values of this type must be evenly divisible by 86_400_000, the number of milliseconds in a standard day.")