Update actions/checkout@v3 -> v4

Author: dtolnay

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
         rust: [nightly, beta, stable, 1.59.0, 1.56.0]
     timeout-minutes: 45
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - 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@v3
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@clippy
       - run: cargo clippy --tests -- -Dclippy::all -Dclippy::pedantic
 
@@ -55,7 +55,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 45
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@miri
       - run: cargo miri setup
       - run: cargo miri test
@@ -68,6 +68,6 @@ 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       - uses: dtolnay/install@cargo-outdated
       - run: cargo outdated --workspace --exit-code 1