DatePicker: Get selectedDay by data instead DOM html

`selectDay()` use jQuery `html()` method on `td` cell to get the day value from DOM, but when user use translator or other extension, it return NaN value because the cell can contains non excepted content. We must use data- attribute to ensure that value cannot be altered.

- https://jqueryui.com/resources/demos/datepicker/localization.html
- https://translate.google.com/translate?hl=en&sl=fr&tl=pl&u=https%3A%2F%2Fjqueryui.com%2Fresources%2Fdemos%2Fdatepicker%2Flocalization.html

We got
```html
<td class=" " data-handler="selectDay" data-event="click" data-month="0" data-year="2021">
    <a class="ui-state-default" href="#">
        <font style="vertical-align: inherit;"><font style="vertical-align: inherit;">22</font></font>
    </a>
</td>
```

instead of
```
<td class=" " data-handler="selectDay" data-event="click" data-month="0" data-year="2021">
   <a class="ui-state-default" href="#">22</a>
</td>
```

so this cannot work correctly
```
 inst.selectedDay = inst.currentDay = $( "a", td ).html();
```

I think it's not a good practice to get value from html DOM element

Author: c-lambert

ui/widgets/datepicker.js

@@ -1036,7 +1036,7 @@ $.extend( Datepicker.prototype, {
 		}
 
 		inst = this._getInst( target[ 0 ] );
-		inst.selectedDay = inst.currentDay = $( "a", td ).html();
+		inst.selectedDay = inst.currentDay = $( "a", td ).data( "date" );
 		inst.selectedMonth = inst.currentMonth = month;
 		inst.selectedYear = inst.currentYear = year;
 		this._selectDate( id, this._formatDate( inst,
@@ -1816,7 +1816,7 @@ $.extend( Datepicker.prototype, {
 							( printDate.getTime() === today.getTime() ? " ui-state-highlight" : "" ) +
 							( printDate.getTime() === currentDate.getTime() ? " ui-state-active" : "" ) + // highlight selected day
 							( otherMonth ? " ui-priority-secondary" : "" ) + // distinguish dates from other months
-							"' href='#'>" + printDate.getDate() + "</a>" ) ) + "</td>"; // display selectable date
+							"' href='#' data-date='" + printDate.getDate() + "'>" + printDate.getDate() + "</a>" ) ) + "</td>"; // display selectable date
 						printDate.setDate( printDate.getDate() + 1 );
 						printDate = this._daylightSavingAdjust( printDate );
 					}