Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/make-coverage-html.yml

@@ -17,7 +17,7 @@ jobs:
     name: "Run make coverage/html and verify coverage HTML"
     runs-on: windows-2025
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Setup .NET
         uses: actions/setup-dotnet@v5
         with: