[EuiDatePicker] Update ARIA and role attributes for better screen reader accessibility

[1Copenut]

Summary

The calendar datepicker needed role="option" added to the individual date choices and the time picker needed an accessible label. Also revised the accessible label for the calendar to closer match the time picker. Closes #5303.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• Props have proper autodocs and playground toggles
• Added documentation
• Checked Code Sandbox works for any docs examples
• Added or updated jest and cypress tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5501/

[chandlerprall]

Changes LGTM, compared this vs. main with mac's voiceover and saw almost no difference due to the verbosity of the live reasons. Did notice that the better-formatted "MMMM, YYYY" is read while tabbing off the calendar listbox.