Specify minimum deployment targets for macOS and iOS to set the minimum OS where our code can be deployed. This at least eliminates a ton of warnings that we would typically see when linking vpnlib into native app. But more than that it probably helps to ensure that our code compiles properly.
Note that to link aarch64 for device/simulator and x8664 simulator I had to resort to using `RUSTFLAGS` since there is no way to make a universal arm64 binary for sim and device since both of them would be aarch64 and that's not allowed but then given individual flags we don't even need a universal binary.