Change default of `SerializationFeature.WRITE_DATES_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 basic Date/Time values as numeric timestamps is something that should be changed: while this is more efficient than the alternative of writing Date/Time value 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!

[cowtowncoder]

Defaults have been changed for 3.0.0-rc1.