ci: bump actions/checkout from 3 to 6 (#1407)

Author: dependabot[bot]

.github/workflows/cpubits.yml

@@ -25,7 +25,7 @@ jobs:
           - stable
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - uses: RustCrypto/actions/cargo-cache@master
       - uses: dtolnay/rust-toolchain@master
         with: