chore(ci): upgrade actions/checkout to v6

Author: paolobarbolini

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - uses: dtolnay/rust-toolchain@stable
         with:
@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - uses: dtolnay/rust-toolchain@stable
         with:
@@ -55,7 +55,7 @@ jobs:
             rust: '1.70'
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - run: sudo $HOME/.cargo/bin/rustup default ${{ matrix.rust }}