update a11y for cookie banner

[carylwyatt]

After conversation with Ange and Gayathri, we decided to update the cookie banner to implement non-modal dialog best practices:

• add a close button
• remove <section> and use a div with role=dialog and aria-modal=false
• implement focus-trapping for keyboard users

With this PR, the banner has been updated to use a close button and focus trap.

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[carylwyatt]

Approved by Ange and Gayathri ✅