Update wasi-sdk to 21.0

Author: kateinoigakukun

.github/workflows/main.yml

@@ -35,7 +35,7 @@ jobs:
         sudo dpkg -i "$wasi_sdk_deb"
         echo "WASI_SDK_PATH=/opt/wasi-sdk" >> $GITHUB_ENV
       env:
-        WASI_SDK_VERSION_MAJOR: 20
+        WASI_SDK_VERSION_MAJOR: 21
         WASI_SDK_VERSION_MINOR: 0
     - run: CARGO_TARGET_WASM32_WASI_RUNNER=wasmtime cargo test --target wasm32-wasi
     - run: cargo build --target wasm32-unknown-unknown