`bevy_picking` feature flag no longer includes `bevy_input_focus`

The bevy/bevy_picking feature flag no longer enables bevy_input_focus picking functionality. For context, bevy_input_focus is inherently a bevy_ui related feature, allowing users to select UI elements to focus using their mouse.

Instead, this functionality is now tied to the existing bevy/ui_picking feature, which is itself part of the ui feature collection. In most cases, you should add the ui feature collection to your project if you are using bevy_ui.

If you want to enable bevy_input_focus's picking functionality, but do not want to use bevy_ui, add a separate dependency to the same version of bevy_input_focus in your project and enable the optional bevy_picking feature there.

This change means it now possible to enable bevy_picking without any assumptions about which backend in particular will be used.