Update actions/checkout@v5 -> v6

Author: dtolnay

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
         rust: [1.76.0]
     timeout-minutes: 45
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{matrix.rust}}
@@ -44,7 +44,7 @@ jobs:
     if: github.event_name != 'pull_request'
     timeout-minutes: 45
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: dtolnay/install@cargo-outdated
       - run: cargo outdated --exit-code 1