[12.x] Align casts property type with casts method

Author: nhedger

src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php

@@ -79,7 +79,7 @@ trait HasAttributes
     /**
      * The attributes that should be cast.
      *
-     * @var array
+     * @var array<string, string>
      */
     protected $casts = [];