This fixes linker warnings that look like this:
```
ld: warning: object file (libswiftCompatibility56.a) was built for newer 'macOS' version (XYZ) than being linked (ABC)
```
These were caused by the compatibility binary being incorrectly built with a newer `-target` than desired: the CMake logic was overriding the requested minimum macOS deployment version (10.9) with a much newer macOS SDK version.
rdar://137565964
When I did https://github.com/swiftlang/swift/pull/74920 I forgot to update test/Concurrency/Backdeploy/linking_maccatalyst.swift the same way linking.swift got updated.
rdar://132710925
These tests were targeting 10.9 or 10.10, but the minimum deployment target now supported by Xcode is 10.13. Bump them up to match:
test/Concurrency/Backdeploy/linking.swift
test/Concurrency/Backdeploy/linking_maccatalyst.swift
test/Runtime/stable-bit-backward-deployment.swift