Document iOS Version Requirement for Core ML Delegate

[GregoryComer]

🚀 The feature, motivation and pitch

We should standardize on a minimum supported iOS version for the Core ML delegate, clearly document it, and add CI to ensure that the delegate runs successfully on the minimum version to catch accidental inclusion of APIs or functionality that require later versions. This has come up in the context of internal adoption. From discussion with existing users, iOS 15 may be a reasonable minimum target.

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

cc @shoumikhin @mergennachin @byjlw

[shoumikhin]

For OSS we set the minimal supported version as 17.0 for iOS and 10.15 for macOS.
It's specified in the build script and SwiftPM package, so the users just can't bypass that and will get errors/warnings when trying to integrate, unless modify and build the runtime locally.

Technically, users may get away with lower versions if they don't use some of the more recent APIs. And those are protected with availability guards, so the users will get proper signals if they try to build the runtime for lower version sand still use more modern features at runtime.

E.g. the lower threshold can be theoretically lowered to 15.4 for MPS and 16.0 for Core ML backends, according to the docs.