fix(cli): disable vertical borders of the update notification

[qwerzl]

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

Fixes #9239. This is a really small modification. I just added a custom borderStyle to boxenOptions.

Test Plan

I changed notifier in beforeCli in beforeCli.mjs to:

updateNotifier({
    pkg: {
      "docusaurus",
      version: '2.4.0',
    },
    // Check is in background so it's fine to use a small value like 1h
    // Use 0 for debugging
    updateCheckInterval: 0,
  });

Test links

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

Screenshot ran in WebStorm with zsh:

Related issues/PRs

#9239

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
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[slorber]

great idea, didn't know it was possible to remove only a part of the border. Thanks 👍