[12.x] feat: use database default datetime precision

[calebdw]

Hello!

This PR updates the default datetime precision to null which in turn uses the database default precision instead of defaulting to 0 across the board.

As I mentioned in #51363 (specifically for Postgres), rounding issues can occur when using a non-default precision for that database. These rounding issues are eliminated when using the database default precision. For example, when using a precision of 0, Postgres will round 2000-01-01 23:59:59.999999 up to the next day (2000-01-02 00:00:00) whereas no rounding will occur when using the default precision (6). At least in Postgres' case, using a lower precision does not have any storage benefits---a timestamp takes up 8 bytes regardless of the precision (however, this may not be the case for all databases).

Note, that if we want more control in the application over the default precision on a per database level, then we could always create getDatetimePrecision methods (similar to getDateFormat) on the schema grammars.

Thanks!

[taylorotwell]

@calebdw can we implement this instead with getDatetimePrecision so that users can just opt-in?