Update actions/checkout@v3 -> v4

Author: dtolnay

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         rust: [nightly, beta, stable]
     timeout-minutes: 45
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{matrix.rust}}
@@ -32,7 +32,7 @@ jobs:
     if: github.event_name != 'pull_request'
     timeout-minutes: 45
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: dtolnay/install@cargo-outdated
       - run: cargo outdated --exit-code 1
         working-directory: _skeptic