[stdlib] Adding @available attribute to Streamable.writeTo(target:)

Fixes [SR-1575](https://bugs.swift.org/browse/SR-1575)

Author: Max Moiseev

stdlib/public/core/OutputStream.swift

@@ -401,3 +401,10 @@ internal struct _TeeStream<
 
 @available(*, unavailable, renamed: "OutputStream")
 public typealias OutputStreamType = OutputStream
+
+extension Streamable {
+  @available(*, unavailable, renamed: "write(to:)")
+  public func writeTo<Target : OutputStream>(target: inout Target) {
+    Builtin.unreachable()
+  }
+}