fix(core): a11y fix on mobile DropdownNavbarItem

[mxschmitt]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

A while ago a focus fix for DropdownNavbarItems was landed (https://github.com/facebook/docusaurus/pull/6003/files) which didn't apply to the mobile version. This mirrors the same logic for the mobile one.

Relates microsoft/playwright.dev#1631

Test Plan

yarn build && yarn start:website and navigate via tabbing over the mobile navbar.

Test links

Deploy preview: https://deploy-preview-10898--docusaurus-2.netlify.app/

Related issues/PRs

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