Update dotnetcore.yml

Author: joadan

.github/workflows/dotnetcore.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        dotnet-version: [6, 7, 8, 9]
+        dotnet-version: [6, 7, 8]
 
     steps:
     - uses: actions/checkout@v2