Text to go in the release notes:
This patch begins the process of replacing an obsolete jQuery plugin with a new library for selecting dates and times. Koha uses the jQueryUI "datepicker" widget for selecting dates, and uses an additional plugin, "jQuery Timepicker Addon," when adding time selection to the widget. This additional plugin has not been updated for many years. The new library, Flatpickr, will eventually replace both the jQuery Timepicker Addon and the jQueryUI datepicker widget. This replacement process begins here with the new Flatpickr calendar widget being added to Circulation -> Renew, Reports -> Patron statistics wizard, and Administration -> Patron categories.