chore: remove swiftLanguageModes declaration

Remove explicit swiftLanguageModes declaration. The code is compatible with both Swift 5 and Swift 6 language modes. The swift-tools-version:6.0 requirement is for Swift Testing framework support, not language mode features.

This provides maximum flexibility for package consumers.

Author: coenttb

Package.swift

@@ -106,8 +106,7 @@ let package = Package(
                 .htmlElementTypesFoundation
             ]
         )
-    ],
-    swiftLanguageModes: [.v6]
+    ]
 )
 
 extension String { var tests: Self { "\(self) Tests" } }