Update swift-argument-parser dependency

[pablocornejo]

Overview

• #178 added Swift 5.6 support, but f38ae33 limited the version flexibility of the swift-argument-parser dependency. The reasoning for this is explained in this comment.
• This makes package resolution for other packages that have Mockolo as a dependency less flexible. So, to have the same flexibility as before f38ae33, but still account for the issue mentioned in the comment linked above, the version range of the swift-argument-parser dependency is being made dependent on the Swift version.