GHA: use .NET 6

Author: janusw

.github/workflows/main.yml

@@ -19,9 +19,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: |
-          3.1.x
-          5.0.x
+        dotnet-version: '6.0.x'
     - name: Install dependencies
       run: dotnet restore src/GeoJSON.Net.sln
     - name: Build
@@ -38,9 +36,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: |
-          3.1.x
-          5.0.x
+        dotnet-version: '6.0.x'
     - name: Install dependencies
       run: dotnet restore src/GeoJSON.Net.sln
     - name: Build