chore: set CI to Swift 6.0 (#109)

Author: danthorpe

.github/workflows/main.yml

@@ -22,14 +22,9 @@ jobs:
         {
           "include": [
             {
-              "os": "macos-14",
-              "swift": "5.9",
-              "xcode": "15.1"
-            },
-            {
-              "os": "macos-14",
-              "swift": "5.10",
-              "xcode": "15.3"
+              "os": "macos-15",
+              "swift": "6.0",
+              "xcode": "16.1"
             }
           ]
         }