CI: Set up NET6 SDK (required by the release-notes tool)

Author: flobernd

.github/workflows/ci.yml

@@ -31,6 +31,8 @@ jobs:
     - uses: actions/setup-dotnet@v4
       with:
         global-json-file: ./global.json
+        dotnet-version: |
+          6.x
         
     - run: ./build.sh build -s true
       name: Build