date-picker: drop keyboardInstruction setting

[elpoelma]

Overview

Since version 1.4.0, duet-date-picker uses basic tables to represent the dropdown calendar. Since screen readers already know how to navigate html tables, the custom keyboardInstruction approach/message is now redundant. See duetds/date-picker#87 for more info.

How to test/reproduce

• Open the app
• Open a page where the date picker is used
• Notice that the calendar used by the date picker is implemented using a basic table

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations