Update actions/checkout to v6

Author: Copilot

.github/workflows/rust.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
     - name: Install lint components
       run: rustup component add rustfmt clippy
     - name: Check formatting