Replace hard-coded year in plain-text markdown emails (#17684)

How plain-text markdown emails have the following header:
```
© 2016 {{ config('app.name') }}. All rights reserved.
```

This changes that to:
```
© {{ date('Y') }} {{ config('app.name') }}. All rights reserved.
```

src/Illuminate/Mail/resources/views/markdown/message.blade.php

@@ -21,7 +21,7 @@
     {{-- Footer --}}
     @slot('footer')
         @component('mail::footer')
-            © 2016 {{ config('app.name') }}. All rights reserved.
+            © {{ date('Y') }} {{ config('app.name') }}. All rights reserved.
         @endcomponent
     @endslot
 @endcomponent