Move to Swift tools version 5.9

Motivation:

The support and test commitment is to work for the last
3 version of swift.  This aligns supported versions with
tested versions.

Modifications:

Move to Swift tools version 5.9 in Package.swift

Result:

Only Swift >=5.9 supported

Author: PeterAdams-A

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.9
 
 import PackageDescription