fix: CodeGen correctly produces yyyy-mm-dd format for date with examples

[Zemke]

When an example was specified for a property with the date format, its example would be generated to something like Sat Jan 30 01:00:00 CET 2021.

It's because the resolved type for a date example is a DateSchema which uses java.util.Date to represents the date causing toString() to not conform to the yyyy-mm-dd format.

This is somewhat analogous to example generation in #17495.

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