GHA (Mac): install .NET SDK 6.0 in addition to 8.0

* in order to fix this error:

Testhost process for source(s) '/Users/runner/work/GeoJSON.Net/GeoJSON.Net/src/GeoJSON.Net.Tests/bin/Debug/net6.0/GeoJSON.Net.Tests.dll' exited with error: You must install or update .NET to run this application.
App: /Users/runner/work/GeoJSON.Net/GeoJSON.Net/src/GeoJSON.Net.Tests/bin/Debug/net6.0/testhost.dll
Architecture: arm64
Framework: 'Microsoft.NETCore.App', version '6.0.0' (arm64)
.NET location: /Users/runner/.dotnet/
The following frameworks were found:
  7.0.2 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]
  7.0.4 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]
  7.0.9 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]
  7.0.20 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]
  8.0.1 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]
  8.0.4 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]
  8.0.7 at [/Users/runner/.dotnet/shared/Microsoft.NETCore.App]

Author: janusw

.github/workflows/main.yml

@@ -36,7 +36,9 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '8.0.x'
+        dotnet-version: | 
+          6.0.x
+          8.0.x
     - name: Install dependencies
       run: dotnet restore src/GeoJSON.Net.sln
     - name: Build