Build is broken on Xcode 16 Beta 1 due to GraphViz

[giginet]

Currently, XcodeGen can't build with Xcode 16 Beta 1.

/Users/giginet/Library/Developer/Xcode/DerivedData/XcodeGen-ccjxvauqmliuzaaqjslkfzkifxrg/SourcePackages/checkouts/GraphViz/Sources/GraphViz/Edge.swift:6:27 'ID' is only available in macOS 10.15 or newer

The following errors were caused by GraphViz. However it is no longer maintained and has already become a public archive.

This dependency is only used in the dump command, so we can easily drop the feature. Therefore, I think it's better to remove this dependency for future maintainability. How do you think?

By the way, I guess this error is caused by a compiler bug. Therefore, it may be resolved by Swift 6 release.