Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/docker.yml

@@ -17,7 +17,7 @@ jobs:
   push:
     runs-on: 'ubuntu-24.04'
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Extract metadata for Docker
         if: github.event_name == 'push' && contains(github.ref, 'refs/tags/')

.github/workflows/main.yml

@@ -23,7 +23,7 @@ jobs:
       DOTNET_CLI_TELEMETRY_OPTOUT: 1
       NUGET_PACKAGES: ${{ github.workspace }}/.github/nuget-packages
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       # Backend
       - name: Set up .NET SDK