[13.x] Support JSON responses for the built-in health route

[WendellAdriel]

The framework's built-in health route always renders HTML, which is awkward for API-only applications whose load balancers, uptime monitors, and orchestrators expect JSON. Today those clients either scrape the HTML or register their own replacement route.

When the incoming request expects JSON ($request->expectsJson()), /up now returns:

{"status": "Application is up"}

or, when a DiagnosingHealth listener throws and debug mode is off:

{"status": "Application experiencing problems"}

Status codes (200 / 500) are unchanged, and requests that don't expect JSON keep getting the existing Blade page.