GH actions - install latest dotnet

.github/workflows/codeql-analysis.yml

@@ -22,6 +22,11 @@ jobs:
       with:
         config-file: ./.github/config/codeql-config.yml
 
+    - name: Setup dotnet
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '6.0.x'
+
     - name: Build
       run: dotnet build umbraco-netcore-only.sln # also runs npm build