The z-coordinate at which to place this app bar relative to its parent.
This property controls the size of the shadow below the app bar if shadowColor is not null.
If surfaceTintColor is not null then it will apply a surface tint overlay to the background color (see Material.surfaceTintColor for more detail).
The value must be non-negative.
If this property is null, then AppBarTheme.elevation of ThemeData.appBarTheme is used. If that is also null, the default value is 4.
This property is used to configure an AppBar.
final double? elevation;