Move frontends action to net5 sdk (testing)

.github/workflows/build-frontends.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.x
+        dotnet-version: 5.0.x
     - name: Install dependencies
       run: dotnet restore Frontends.sln
     - name: Build