fix(core): always treat error boundary fallback as a callback

[Josh-Cena]

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

Re-do #7368, after @slorber pointed out about performance issues.

The problem still stands: we needs to be clear whether the prop is a callback to be called, or a component to be rendered with JSX. We cannot have a mix of both.

Test Plan

Test links

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

Related issues/PRs

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 84	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 85	🟢 99	🟢 100	🟢 100	🟢 90	Report

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[github-actions]

Size Change: +98 B (0%)

Total Size: 796 kB

Filename	Size	Change
website/build/assets/js/main.********.js	598 kB	+98 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/index.html	38.9 kB

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