[Fix] DateInput aria-selected

[LJKaski]

Description

This PR adds selected value or corresponding translation in the aria-label of the button representing the selected date in DatePicker. It also addresses an issue where a redundant aria-disabled=false attribute would be present in DOM for most day selectors in date picker.

Motivation and Context

This issue came up in discussions and is an accessibility oversight that needed to be fixed.

How Has This Been Tested?

Tested in styleguidist on chrome with NVDA and TalkBack and by checking the DOM.

Release notes

DateInput

• Add selected date or corresponding translation for the selected date button in date picker
• Remove redundant aria-attributes from DOM

[danielck]

Unfortunately role="option" can only be used inside a listbox or group. After reviewing a few other implementations and options, semantically it seems the best bet is to do the following:

• Set the tableto role="grid". This will allow us to use aria-selected later. A grid must have an accessible name, so it needs an aria-label. This can be simply the month + year.
• Set role="gridcell" on individual tdelements, however this might not strictly be necessary according to the spec.
• Set aria-selected="true" on the wrapping td, not the button.
• Because there seems to be crappy support for everything mentioned above, I suggest adding "selected" to the aria label of the selected date.