fix(core): fix configurePostCss v3.2 regression

[slorber]

Motivation

My v3.2 refactors led to a regression, and configurePostCss was wrongly applied to the Webpack server config instead of client config.

Fix #10005

Test Plan

none 😅 I'll probably have to refactor things more to make this easier to test, for now I'm just fixing the regression asap.

Test links

https://deploy-preview-10012--docusaurus-2.netlify.app/

[netlify]

✅ [V2]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 72	🟢 98	🟢 100	🟢 100	🟠 88	Report
/docs/installation	🟠 59	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 71	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 70	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: 0 B

Total Size: 1.05 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	91.2 kB
website/build/assets/css/styles.********.css	114 kB
website/build/assets/js/main.********.js	809 kB
website/build/index.html	38 kB

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