ci: Update actions/checkout to v4 (WebAssembly#73)

Author: waywardmonkeys

.github/workflows/main.yml

@@ -10,5 +10,5 @@ jobs:
     name: Check ABI files are up-to-date
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: WebAssembly/wit-abi-up-to-date@v16