Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/BuildOnly.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
     - name: Checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
        fetch-depth: 0 # avoid shallow clone so nbgv can do its work.
 

.github/workflows/dotnet.yml

@@ -16,7 +16,7 @@ jobs:
       nbgv: ${{ steps.nbgv.outputs.SemVer2 }}
     steps:
     - name: Checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
        fetch-depth: 0 # avoid shallow clone so nbgv can do its work.