fix(core): update webpack-dev-server + fix deprecation warning

[slorber]

Motivation

Webpack just deprecated an option that we use onBeforeSetupMiddleware and added a warning: webpack/webpack-dev-server#4129

Fix #6167

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

local, works without any warning

[netlify]

✔️ [V2]

🔨 Explore the source changes: 2b131ff

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61c37309a38657000882531e

😎 Browse the preview: https://deploy-preview-6168--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	72
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-6168--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 662 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	40.1 kB
website/build/assets/css/styles.********.css	102 kB
website/build/assets/js/main.********.js	490 kB
website/build/index.html	29.6 kB

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