Update github actions Swift versions to latest

Author: jcavar

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
     with:
       scheme: AudioKitEX
       platforms: iOS macOS tvOS
-      swift-versions: 5.5 5.6
+      swift-versions: 5.9
 
   # Send notification to Discord on failure.
   send_notification: