fix: rename middleware.ts to proxy.ts

The middleware file convention is deprecated in this Next.js version in favor of proxy.

src/proxy.ts

@@ -0,0 +1,10 @@
+import createMiddleware from 'next-intl/middleware'
+import { routing } from './i18n/routing'
+
+export default createMiddleware(routing)
+
+export const config = {
+  matcher: [
+    '/((?!api|_next|_vercel|.*\\..*).*)',
+  ],
+}