ci: test in net6

Author: ikesnowy

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   test-net6:
     runs-on: ubuntu-latest
-    container: mcr.microsoft.com/dotnet/sdk:8.0
+    container: mcr.microsoft.com/dotnet/sdk:6.0
 
     steps:
       - name: Checkout