Remove aria-hidden attributes from focusables

[compulim]

Related to #4652.

Changelog Entry

Fixed

• Fixes #4652. Keyboard help screen, activity focus traps, and chat history terminator should not be hidden behind aria-hidden because they are focusable, by @compulim, in PR #4659

Description

Focusables (e.g. tabindex="0") should not be hidden behind aria-hidden.

For tests, we will be adding axe-core in #4651 and I verified that it would catch these accessibility issues.

Specific Changes

• Remove aria-hidden in focus traps
• Remove aria-hidden for keyboard help screen
  • It is okay to read the help screen even though it is not on-screen
• Remove aria-hidden for chat history terminator
  • It is okay to read "end of chat history"

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)