[EuiCollapsibleNavBeta] A11y improvements

[cee-chen]

(Follow up to #6759)

Since the newly redesigned EuiCollapsibleNav will be used on every single serverless Kibana page going forward, we should take extra steps to make sure its accessibility is top-notch for screen reader, keyboard, and mouse users.

This issue is a punch list of remaining items to address for the new nav experience:

• (Push/collapsed mode)

  • EuiButtonIcons are missing aria-labels (this is because the tooltips repeat the labels as aria-describedby so
    we need some way to un-duplicate that information)
  • Resolved by [EuiCollapsibleNavBeta] Improve collapsed button icon screen reader UX #7055

• Nav items with accordion initialIsOpen are incorrectly tabbable ([EuiAccordion] Children are still present in keyboard tab order when accordions are closed #6980)

  • Resolved by [EuiAccordion] Improve keyboard accessibility of tabbable children #7064

• (Push/desktop modeTwo invisible focus stops exist between the flyout toggle button and the flyout contents

  • Partially resolved by [EuiFlyoutBody] Add scrollableTabIndex prop #7061
  • Partially resolved by [EuiFlyout] Resolve disparity between push flyouts and overlay flyouts #7065

[ ] Ability to skip entire nav on pushed/desktop mode Scrapping this for now, I think the default EuiSkipLink at the top of the page suffices