Bump swift-syntax version range

[davdroman]

Your package is resolving Swift Syntax as 600.0.1 in my app. I think something else needs tweaking cause I'd expect it to be 602 (or 601 at least).

[davdroman]

Also not to tell you what to do but I don't think anybody expects Swift 5.9 compatibility these days, especially since your library is still 0.x. I think 600.0.0..<603.0.0 as a blanket version range is perfectly acceptable.

[authiatr]

Thank you @davdroman for your contribution, that's exactly what I was looking for.
I'm facing some conflicts with external dependencies in Swift-Syntax; your changes are solving them.
I hope it can get merged soon 🙌

[davdroman]

Superseded by #53