[CI] Update the checkout action because Node 20 is deprecated.

Author: hageboeck

.github/workflows/build-exercises.yml

@@ -97,7 +97,7 @@ jobs:
     # The build/test steps to execute.
     steps:
     # Use a standard checkout of the code.
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     # Run the CMake configuration.
     - name: CMake Configure
       run: cmake -S ${{ github.workspace }}/exercises/${{ matrix.EXERCISE.NAME }}

.github/workflows/build-slides.yml

@@ -21,7 +21,7 @@ jobs:
         version: [ essentials, full ]
     steps:
       - name: Set up Git repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Compile essentials
         uses: xu-cheng/latex-action@v3
         if: matrix.version == 'essentials'

.github/workflows/publish-slides.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         version: [ essentials, full ]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Compile Essentials document
         if: matrix.version == 'essentials'
         uses: xu-cheng/latex-action@v3