Bump swift-tools-version to 6.0

Since we're no longer directly testing with Swift 5.9 and 5.10, let's try to bump `swift-tools-version` to 6.0 to declare that these older versions are no longer supported.

Author: MaxDesiatov

Package.resolved

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 6.0
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
@@ -116,8 +116,7 @@ let package = Package(
         .product(name: "Logging", package: "swift-log"),
       ]
     ),
-  ],
-  swiftLanguageVersions: [.v5, .version("6")]
+  ]
 )
 
 struct Configuration {