[vercel-tutor] Upgrade Next.js and Tailwind (#1099)

### Description

Upgrades Next.js to 15.2.0 and Tailwind to 4.0 on `vercel-tutor`.

### Demo URL

<!--
Provide a URL to a live deployment where we can test your PR. If a demo
isn't possible feel free to omit this section.
-->

### Type of Change

- [ ] New Example
- [ ] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [ ] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [ ] 📱 Is it responsive? Are mobile and tablets considered?

vercel-tutor/app/globals.css

@@ -1,6 +1,10 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
+
+@theme {
+  --color-background: var(--background);
+  --color-foreground: var(--foreground);
+  --color-secondary: var(--secondary);
+}
 
 :root {
   --background: #ffffff;

vercel-tutor/package.json

@@ -10,18 +10,18 @@
   "dependencies": {
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
-    "next": "15.1.3"
+    "next": "15.2.0"
   },
   "devDependencies": {
     "@eslint/eslintrc": "^3",
+    "@tailwindcss/postcss": "^4",
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "eslint": "^9",
-    "eslint-config-next": "15.1.3",
-    "postcss": "^8",
+    "eslint-config-next": "15.2.0",
     "prettier": "^3.4.2",
-    "tailwindcss": "^3.4.1",
+    "tailwindcss": "^4",
     "typescript": "^5"
   }
 }