OS support policy

[christiangnrd]

I noticed we still mention the possibility of supporting macOS older than macOS 13 in the readme.

Metal.jl/README.md

Lines 39 to 45 in 2ff52d6

	These requirements are fairly strict, and are due to our limited development
	resources (manpower, hardware). Technically, they can be relaxed. If you are
	interested in contributing to this, see [this
	issue](https://github.com/JuliaGPU/Metal.jl/issues/22) for more details.
	In practice, Metal.jl will probably work on any macOS 10.15+, and other
	GPUs that are supported by Metal might also function (if only partially),
	but such combinations are unsupported for now.

Apple only supports 3 versions. For example macOS 13 will probably stop being supported once macOS 16 is released in the Fall. Do we want to follow that schedule? If so, do we leave the compatibility code indefinitely until it naturally breaks or do we explicitly clean up the code once we stop supporting a version?

Somewhat related to #22.

[maleadt]

Following the vendor seems sane, and is what we do in CUDA.jl too. Since Pkg is unaware of OS versions though, we can't express compatibility, so it'd be good to maintain a table in the README (If on macOS X, use Metal.jl Y) + a check at initialization or first use.

do we leave the compatibility code indefinitely until it naturally breaks or do we explicitly clean up the code once we stop supporting a version

I'd emit a warning, and let the user continue at his own peril.