libretro: restore requested vulkan api version to match VulkanLoader

[warmenhoven]

Changed by #19857 , this breaks loading the ppsspp core on devices where the Vulkan api provided is 1.2 (or lower), particularly Apple devices.

[hrydgard]

Why not query it like we do in standalone?

[warmenhoven]

Retroarch actually does do vkEnumerateInstanceVersion, and does try to upgrade it if possible. So really the libretro api is expecting the minimum supported version, not the maximum. Actually the libretro docs even say that's the case:

* If app is non-NULL, apiVersion controls the minimum core version required by the application.

app here refers to get_application_info which is a function that returns the VkApplicationInfo, and core is the vulkan core.