[EuiDatePicker][COGNITION]: Add an `aria-live="polite"` attribute to the timestamp string container

[1Copenut]

Description

Our EuiDatePicker component ( https://eui.elastic.co/#/forms/date-picker ) has a aria-live="off" attribute on the top-level DIV. This appears to be a change from the past where it was assertive. We still need to let users know something changed, but it would be better to announce the timestamp string by adding a live region around the container. Screenshot below.

I'm proposing we move the live region to wrap the calendar and time picker only, retest with screen readers (JAWS, NVDA, VO) and re-evaluate if the experience is better, worse, or about the same.

Screenshot below.

Guidance

• https://www.w3.org/WAI/WCAG21/Understanding/meaningful-sequence.html
• https://www.w3.org/WAI/WCAG21/Understanding/status-messages.html

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