Fix .net version for test github action

Author: Sergi0Martin

.github/workflows/open-mediator-nuget-package.yml

@@ -43,6 +43,10 @@ jobs:
     needs: [build]
     steps:
     - uses: actions/checkout@v4
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v2
+      with:
+        dotnet-version: 9.0.x
     - name: Test
       run: dotnet test OpenMediator.sln
     - name: Test