Make CommandParser.swift import Swift version consistent with other files. (#724)

Author: rgoldberg

Sources/ArgumentParser/Parsing/CommandParser.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if swift(>=5.11)
+#if swift(>=6.0)
 internal import class Foundation.ProcessInfo
 #else
 import class Foundation.ProcessInfo