Update docusaurus for dependabot (#10404)

Addresses
https://github.com/buildbuddy-io/buildbuddy/security/dependabot/223

Also upgraded node since the newest docusaurus requires node 20.

Author: tempoz

website/package.json

@@ -13,12 +13,12 @@
     "@algolia/client-search": "^5.15.0",
     "@cmfcmf/docusaurus-search-local": "1.2.0",
     "@docsearch/react": "^3.8.0",
-    "@docusaurus/core": "^3.6.3",
-    "@docusaurus/plugin-client-redirects": "^3.6.3",
-    "@docusaurus/plugin-content-blog": "^3.6.3",
-    "@docusaurus/plugin-content-docs": "^3.6.3",
-    "@docusaurus/plugin-ideal-image": "^3.6.3",
-    "@docusaurus/preset-classic": "^3.6.3",
+    "@docusaurus/core": "^3.9.1",
+    "@docusaurus/plugin-client-redirects": "^3.9.1",
+    "@docusaurus/plugin-content-blog": "^3.9.1",
+    "@docusaurus/plugin-content-docs": "^3.9.1",
+    "@docusaurus/plugin-ideal-image": "^3.9.1",
+    "@docusaurus/preset-classic": "^3.9.1",
     "@mdx-js/react": "^3.0.0",
     "@types/react": "^18.3.12",
     "cheerio": "^1.0.0",