Github Actions: install .NET 5.0

* in addition to .NET Core 3.1
* and use the latest version of the 'setup-dotnet' action

Author: janusw

.github/workflows/main.yml

@@ -16,9 +16,11 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Setup .NET Core
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 3.1.301
+        dotnet-version: |
+          3.1.x
+          5.0.x
     - name: Install dependencies
       run: dotnet restore src/GeoJSON.Net.sln
     - name: Build