Don't force Vulkan 1.3

We are currently [forcing Vulkan 1.3](https://github.com/llvm-beanz/offload-test-suite/blob/main/test/lit.cfg.py#L56), which isn't ideal since we should really be able to write tests against any version of Vulkan.

For now the non-1.3 support in LLVM is pretty limited, so this might be the right call temporarily, but we should figure out a better long term solution.