ci: Bump GitHub checkout action to v6

Author: xavier2k6

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install GCC ${{ matrix.version }}
         run: sudo apt-get install -y gcc-${{ matrix.version }} g++-${{ matrix.version }}
@@ -60,7 +60,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install Clang ${{ matrix.version }}
         run: sudo apt-get install -y clang-${{ matrix.version }}
@@ -91,7 +91,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Configure tests
         run: cmake -S . -B build