Update to actions/checkout@v6

Author: gknowles

.github/workflows/github-build.yml

@@ -62,7 +62,7 @@ jobs:
         echo "C:\Program Files\OpenCppCoverage" >> %GITHUB_PATH%
 
     - name: Checkout
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
       with:
         submodules: true