chore: consolidate to Swift 6 language mode and remove versioned manifest

- Update swiftLanguageModes from .v5 to .v6
- Remove [email protected] (no longer needed with Swift 6.0 minimum)

With Swift 6.0 as the minimum version, we can use Swift 6 language mode directly and don't need separate package manifests.

Author: coenttb

Package.swift

@@ -107,7 +107,7 @@ let package = Package(
             ]
         )
     ],
-    swiftLanguageModes: [.v5]
+    swiftLanguageModes: [.v6]
 )
 
 extension String { var tests: Self { "\(self) Tests" } }