FindVulkan: Add policy to remove this module

Our FindVulkan module was written for one Vulkan SDK distribution and is not compatible with other Vulkan distributions. It also finds some of Vulkan's related components as if they were part of the Vulkan package, making it incompatible with direct use of those individual packages. Add a policy to pretend the module doesn't exist in order to encourage implementation of alternatives externally.

Closes: #25617