Upgrade to Swift 6 compiler (Swift 5 language mode)

- Darwin platform
    - https://github.com/OpenSwiftUIProject/OpenGraph/pull/80
- Linux platform
    - https://github.com/OpenSwiftUIProject/OpenGraph/pull/81
- WASM platform
    - [x] Blocked by [upstream release](https://github.com/swiftwasm/swift/issues/5582)