Recommend objc2-metal instead of metal

[grovesNL]

The metal crate has been around for a while now and was historically maintained by the teams of gfx pre-ll/gfx-hal (legacy) and wgpu (current). wgpu is planning to switch over to objc2 though, so that will remove one of the biggest maintainers of this crate.

As a maintainer, I'm happy to continue maintaining metal where it makes sense, but I'd like to start recommending that people switch over to obj2-metal. I'd like to add this at the beginning of the README to start directing people there. Many of the other crates in the ecosystem have already started to switch over (e.g., winit uses objc2), so I'd like to help push this forward while giving people some time to gradually transition from metal.

If anyone has concerns about recommending objc2-metal and metal maintenance, let's discuss them here.

[grovesNL]

@kvark would you be ok with this?