Mention the replacement methodd in deprecated attributes

[DethRaid]

vk-bootstrap has deprecated vkb::PhysicalDeviceSelector::add_desired_extension, but the method we should call instead is not obvious. Luckily, C++'s deprecated attribute lets you add a string to describe the new method or functionality. I'd like to request adding this string to the deprecated attributes on vkb::PhysicalDeviceSelector::add_desired_extension saying to use vkb::PhysicalDevice::enable_extension_if_present