Bump up the macOS platform to .macOS(.v15) in Package.swift & Package@swift-6.0.swift

[IgorMuzyka]

Bug
Currently both Package.swfit and Package@swift-6.0.swift state platforms as .macOS(.v13).
The following lines in Execution.swift raise errors: 49, 82, 96, 97, 99, 120, 121, 123.
The errors are:

'X' is only available in macOS 15.0 or newer

To Reproduce
Just add this package as dependency to another package and it's target and see build fail.

Expected behavior
Correct platform version.

Environment (please complete the following information):

• macOS 15.4.1
• Xcode 16.3
• Swift version swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3) Target: arm64-apple-macosx15.0

[fpseverino]

If this library could also work on macOS 13 that would be ideal, would that be possible?

There are a lot of projects, CLI tools and various libraries, that need to support macOS 13 and would benefit greatly from using the new Subprocess

[iCharlesHu]

@fpseverino yes that's the goal. See #4