GHA: use .NET 8 SDK

Author: janusw

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '6.0.x'
+        dotnet-version: '8.0.x'
     - name: Install dependencies
       run: dotnet restore src/GeoJSON.Net.sln
     - name: Build
@@ -36,7 +36,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '6.0.x'
+        dotnet-version: '8.0.x'
     - name: Install dependencies
       run: dotnet restore src/GeoJSON.Net.sln
     - name: Build