Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/docker.yml

@@ -16,7 +16,7 @@ jobs:
   push:
     runs-on: 'ubuntu-22.04'
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Build and Push Docker Images
         uses: docker/build-push-action@v1

.github/workflows/main.yml

@@ -22,7 +22,7 @@ jobs:
       DOTNET_CLI_TELEMETRY_OPTOUT: 1
       NUGET_PACKAGES: ${{ github.workspace }}/.github/nuget-packages
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       # Backend
       - name: Set up .NET SDK