fix: Fix invalid DOM property tabindex error (#2809)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: tjuanitas

src/components/date-picker/DatePicker.tsx

@@ -622,7 +622,7 @@ class DatePicker extends React.Component<DatePickerProps, DatePickerState> {
 
         // De-emphasizing the Pikaday date picker because it does not meet accessibility standards
         // Screenreaders & navigating via keyboard will no longer pick up on this element
-        const accessibleAttrs = isAccessible ? { 'aria-hidden': true, tabindex: -1 } : {};
+        const accessibleAttrs = isAccessible ? { 'aria-hidden': true, tabIndex: -1 } : {};
 
         return (
             <PlainButton

src/components/flyout/Flyout.js

@@ -389,7 +389,7 @@ class Flyout extends React.Component<Props, State> {
             onMouseEnter: this.handleButtonHover,
             onMouseLeave: this.handleButtonHoverLeave,
             role: 'button',
-            tabindex: '0',
+            tabIndex: '0',
             'aria-haspopup': 'true',
             'aria-expanded': isVisible ? 'true' : 'false',
         };