ci: Bump actions/checkout to the latest version (v6)

Author: tgross35

.github/workflows/main.yaml

@@ -29,7 +29,7 @@ jobs:
             os: ubuntu-24.04
             rust: nightly
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
     - name: Install Rust
       run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
     - name: Build and run tests
@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-24.04
     timeout-minutes: 10
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
     - name: Install Rust
       run: rustup update stable && rustup default stable && rustup component add rustfmt
     - run: cargo fmt --check