Generate macOS platform tag based on the dylib build version

[messense]

Currently it's hard coded if MACOSX_DEPLOYMENT_TARGET env var isn't set

maturin/src/target.rs

Lines 685 to 686 in cb0b6c6

	let x86_64_default = if universal2 { (10, 9) } else { (10, 7) };
	let arm64_default = (11, 0);

[messense]

I think in reality it's not a good idea to do this because it will end up generating wheels that can only be installed on newer versions of macOS.

Always setting MACOSX_DEPLOYMENT_TARGET is still a good idea, but I think we should add a option in [tool.maturin] like macos-deployment-target to allow projects to declare a default target version, instead of relying on querying the one that rustc support because there might be some C/C++ dependencies that requires higher macOS SDK versions than rustc.