fix: microsite/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-POSTCSS-5926692

Author: snyk-io[bot]

microsite/package.json

@@ -31,7 +31,7 @@
   "dependencies": {
     "@docusaurus/core": "0.0.0-5591",
     "@docusaurus/plugin-client-redirects": "0.0.0-5591",
-    "@docusaurus/preset-classic": "0.0.0-5591",
+    "@docusaurus/preset-classic": "3.8.1",
     "@swc/core": "^1.3.46",
     "clsx": "^1.1.1",
     "docusaurus-plugin-sass": "^0.2.3",