Dump to latest .NET SDK [skip ci]

Author: timheuer

.github/workflows/build.yml

@@ -28,9 +28,9 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Setup .NET Core SDK
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v1.8.0
       with:
-        dotnet-version: 3.1.x
+        dotnet-version: 5.0.x
 
     - name: Restore
       run: dotnet restore
@@ -81,7 +81,7 @@ jobs:
       - name: Setup .NET Core SDK
         uses: actions/setup-dotnet@v1.8.0
         with:
-          dotnet-version: 3.1.x
+          dotnet-version: 5.0.x
 
       - name: Get certificate
         id: cert_file