fix calendar weekday name with custom styles

[atarek12]

Description

Linked issue: #(issue number)

Problem
There is an issue with weekday name when using datepicker component without using datepicker styles
i.e: import "react-datepicker/dist/react-datepicker.css";

Changes
The issue has been introduced in this pr that fixes this issue.
But instead of using sr-only class on an extra span element which will be rendered if we don't import the stylesheet.
I have reverted it back to use aria-label but kept the inner span with aria-hidden="true"; so the screen-readers will skip it and announce the aria-label prop.

Screenshots

• Before: Notice the duplicate of weekday -> SundaySu

• After

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.

[martijnrusschen]

Please check the output of the failing CI