FindVulkan: Remove this find module

The FindVulkan module can only find the Vulkan SDK from LunarG, but there are other ways to install Vulkan. Also, several problems have come up with FindVulkan's design, discussed in other issues and MRs, including:

#25384 (incompatible with Vulkan upstream CMake packages)
#25583 (closed) (uses private glslang components)
!9158 (comment 1471186) (suggestion to deprecate)
Vulkan SDK issue (requests cmake package files in SDK)

The FindVulkan module finds several components from the Vulkan SDK all under the "Vulkan" umbrella even though some of them are dependencies. This makes the module incompatible with other Vulkan distributions and with use of the dependencies as their own packages.

I plan to remove the FindVulkan module via a policy to make room for alternatives such as the upstream CMake package files. Projects that want to find the Vulkan SDK as before can vendor a copy of the old FindVulkan module themselves.

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