fix(next): persist language cookie across browser sessions (#15081)

### What?

Persist the language selection cookie by setting a max age so it
survives browser restarts.

### Why?

The language cookie was session-only, so closing the browser reset the
language preference.

### How?

Add `maxAge` to the language cookie set in `switchLanguageServerAction`
within `packages/next/src/layouts/Root/index.tsx`.

Author: Robot-Inventor

packages/next/src/layouts/Root/index.tsx

@@ -102,6 +102,7 @@ const RootLayoutContent = async ({
     const cookies = await nextCookies()
     cookies.set({
       name: `${config.cookiePrefix || 'payload'}-lng`,
+      maxAge: 60 * 60 * 24 * 365,
       path: '/',
       value: lang,
     })