Replace @retroactive annotation with fully qualified type name of protocol to support pre 6.0 compilers

Using fully qualified type names in retroactive conformances is viewed as equivalent
to a @retroactive annotation by Swift 6 compilers, specifically for the usecase of
writing backwards compatible code.

Author: stackotter

Sources/GtkCodeGen/Extensions.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-extension String: @retroactive CodingKey {
+extension String: Swift.CodingKey {
     public init?(intValue: Int) {
         return nil
     }