hello_xr vulkan backend ignores `minApiVersion` from graphics requirements structs

[fredemmott]

VK 1.0 is hardcoded here: https://github.com/KhronosGroup/OpenXR-SDK-Source/blob/820278f99f98928ac1f3d27916c4aa24c6539521/src/tests/hello_xr/graphicsplugin_vulkan.cpp#L1386

For XR_KHR_vulkan_enable2, the version can (should) be forced by the runtime or an API layer's implementation of xrCreateVulkanInstanceKHR, however this isn't supported via XR_KHR_vulkan_enable - for the older OpenXR extension, the app should respect the min api version from XrGraphicsRequirementsVulkanKHR and use it for vkCreateInstance()