Upgrade flask to ^2.3.0

This required chaning the patching of jsonifying of datetime/date values to use ISO format.

As of 2.2.*, changing the datetime and date JSON encoding is done globally,
see https://stackoverflow.com/a/74618781/2692895 (which is an updated reply to
https://stackoverflow.com/questions/43663552/keep-a-datetime-date-in-yyyy-mm-dd-format-when-using-flasks-jsonify)

Author: michael.yak