Update Package.swift for Swift 5.6 release.

[thii]

No description provided.

[circAssimilate]

Is there anything I can do to help with this? For now, I've just checked out this branch while we wait for the 5.6 release.

[michaellenaghan]

Without this patch, or something like it, building the 5.6 branch using Xcode 13.3 produces this error:

[0/1] Planning builderror: the executable 'swift-format' requires macos 10.11, but 
depends on the product 'ArgumentParser' which requires macos 10.15; consider 
changing the executable 'swift-format' to require macos 10.15 or later, or the product 
'ArgumentParser' to require macos 10.11 or earlier.

[allevato]

As of right now, we're going to need https://bugs.swift.org/browse/SR-15989 to be resolved before we can release a 5.6 compatible version, to fix the SPM dependency issue. swiftlang/swift-syntax#372 should fix that; they'll need to cut a new release after that.

[kylebrowning]

Looks like this should be ready @allevato

[allevato]

This has been superseded by #286 (to use the newly tagged 0.50600.1 swift-syntax release and a different swift-argument-parser release.

I'll be tagging the new swift-format release momentarily.