hello_xr vulkan backend ignores `minApiVersion` from graphics requirements structs

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()`

Start in src/tests/hello_xr/graphicsplugin_vulkan.cpp at the hardcoded VK 1.0 value around line 1386. Trace the graphics requirements data for XR_KHR_vulkan_enable and verify that minApiVersion is used when calling vkCreateInstance; done means the hello_xr Vulkan backend no longer ignores that requirement.

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