chore: remove docusaurus-migrate

[slorber]

Motivation

This docusaurus-migrate package was created for the v1 => v2 transition and doesn't really make sense anymore to upgrade to newer Docusaurus versions.

We could create a helper CLI for upcoming upgrades, but that would be a totally different tool rewritten from scratch. In the meantime we can safely delete this package.

Test Plan

CI

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	5bd8ec8
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/652917195b6c3200083460c4
😎 Deploy Preview	https://deploy-preview-9400--docusaurus-2.netlify.app
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 70	🟢 97	🟢 92	🟢 100	🟠 89	Report
/docs/installation/	🟠 71	🟢 98	🟢 92	🟢 100	🟠 89	Report
/docs/category/getting-started/	🟠 83	🟢 100	🟢 92	🟢 90	🟠 89	Report
/blog/	🟠 76	🟢 100	🟢 92	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3/	🟠 75	🟢 97	🟢 92	🟢 100	🟠 89	Report
/blog/tags/release/	🟠 86	🟢 100	🟢 92	🟠 80	🟠 89	Report
/blog/tags/	🟠 76	🟢 100	🟢 92	🟢 90	🟠 89	Report

[github-actions]

Size Change: 0 B

Total Size: 1.13 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	117 kB
website/build/assets/css/styles.********.css	113 kB
website/build/assets/js/main.********.js	861 kB
website/build/index.html	41.4 kB

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