Datepicker: Clean up and modernize validations

[olemartinorg]

Description

The Datepicker component suffers from validation duplication, as it keeps validation messages both locally and in Redux. The function validateDatepickerFormData is called both in validation.ts and in DatepickerComponent - but we should aim to keep the validation state in redux only.

Additional Information

• DatePicker validations for minDate/maxDate not working in repeating groups #185
• Validation: Hide error message when the content is corrected by user #98
• Architecture: Modularize components using classes and inheritance #377

Tasks

• Remove duplicate/local validation in DatepickerComponent.tsx, move missing validation into validation.ts
• Implement delayed saving with useDelayedSavedState if useful
• Make sure required field validation does not run immediately if field is empty (should only run when clicking next)
• Make sure validation works when Datepicker is in a repeating group

Acceptance Criterias

No response

[olemartinorg]

Tested, working wonderfully!