Use @retroactive in Swift 6.0 compiler and newer only (#919)

* Use @retroactive in Swift 6.0 and newer only

* Use @retroactive in Swift 5.10 and newer only

* Use @retroactive in Swift 6.0 compiler and newer only

Author: tinder-cfuller

Sources/NodesGenerator/YAMLDecoder.swift

@@ -11,7 +11,7 @@ import Codextended
 import Yams
 
 // Enables Codextended with YAMLDecoder [https://github.com/JohnSundell/Codextended]
-#if swift(>=5.9)
+#if compiler(>=6.0)
 extension YAMLDecoder: @retroactive AnyDecoder {}
 #else
 extension YAMLDecoder: AnyDecoder {}