Update package version

[0111b]

When XcodeGen used as a dependency in the project with same dependencies package definition became invalid.
This PR fixes this issue.

Fixes #1490

[yonaskolb]

@0111b What's the swift version of the package you're importing XcodeGen into? We can probably update the swift version in the xcodegen package to something like 5.9 at this point

[0111b]

@yonaskolb I'm using exactly Swift 5.9. I didn't think about backward compatibility.
Should I create a separate Package@swift-5.9.swift ?

[yonaskolb]

At this point I'm fine with dropping older versions, so no need for Package@swift-5.9.swift Can you make that change here. I'll handle the release notes after merging

[0111b]

@yonaskolb I've bumped swift tools version in the package manifest

[0111b]

It looks like CI failed due API rate limit issue

[yonaskolb]

Thanks @0111b