Change default of `SerializationFeature.WRITE_DURATIONS_AS_TIMESTAMPS` to `false` in 3.0

[cowtowncoder]

(note: part of proposed changes via https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-2)

It seems that the default of writing "Duration" (like Period) values as numeric timestamps is something that should be changed: while this can be more efficient than the alternative of writing Duration values as Strings, most users expect Strings, and most frameworks seem to change the default.

So the proposal is to change the default accordingly.

[cowtowncoder]

Change has been approved: we can proceed with change!