Potentially drop support for `vk::UniqueHandles` in the future

As shortly mentioned in https://github.com/KhronosGroup/Vulkan-Hpp/pull/2358#issuecomment-3885748285, use cases for `vk::UniqueHandle` could be superceded by the more versatile `vk::raii::Handles`. This might warrant deprecating them and eventually removing them from the headers.

One concern I wrote about in the abovementioned thread is that `vk::UniqueHandles` use the normal static/dynamic dispatchers, whereas `vk::raii::Handles` apparently use their own. The former could therefore be much more easily mixed together with the normal `vk::Handles`. It would be great if someone with more experience using the raii handles could comment on this.

Start with the linked pull request comment and compare the documented use of vk::UniqueHandles with vk::raii::Handles, focusing on their static and dynamic dispatcher behavior. Gather evidence about whether the two handle styles can be mixed safely; done requires a maintainer-backed decision on deprecation or removal, rather than a narrowly specified code change.

Written by the indexing model from the issue text.