Wrap SignalProducer protocol declarations in a swift version check

Sources/SignalProducer.swift

@@ -538,6 +538,7 @@ extension SignalProducer where Error == Swift.Error {
 	}
 }
 
+#if swift(>=5.7)
 /// Represents reactive primitives that can be represented by `SignalProducer`.
 public protocol SignalProducerConvertible<Value, Error> {
 	/// The type of values being sent by `self`.
@@ -549,7 +550,21 @@ public protocol SignalProducerConvertible<Value, Error> {
 	/// The `SignalProducer` representation of `self`.
 	var producer: SignalProducer<Value, Error> { get }
 }
+#else
+/// Represents reactive primitives that can be represented by `SignalProducer`.
+public protocol SignalProducerConvertible {
+	/// The type of values being sent by `self`.
+	associatedtype Value
+
+	/// The type of error that can occur on `self`.
+	associatedtype Error: Swift.Error
+
+	/// The `SignalProducer` representation of `self`.
+	var producer: SignalProducer<Value, Error> { get }
+}
+#endif
 
+#if swift(>=5.7)
 /// A protocol for constraining associated types to `SignalProducer`.
 public protocol SignalProducerProtocol<Value, Error> {
 	/// The type of values being sent by `self`.
@@ -561,6 +576,19 @@ public protocol SignalProducerProtocol<Value, Error> {
 	/// The materialized `self`.
 	var producer: SignalProducer<Value, Error> { get }
 }
+#else
+/// A protocol for constraining associated types to `SignalProducer`.
+public protocol SignalProducerProtocol {
+	/// The type of values being sent by `self`.
+	associatedtype Value
+
+	/// The type of error that can occur on `self`.
+	associatedtype Error: Swift.Error
+
+	/// The materialized `self`.
+	var producer: SignalProducer<Value, Error> { get }
+}
+#endif
 
 extension SignalProducer: SignalProducerConvertible, SignalProducerProtocol {
 	public var producer: SignalProducer {