Fix gcc warnings in mono (#62684)

Fix gcc warnings in mono (#62684)

* Fix gcc warnings in mono

* Cleanup trailing whitespace in changeset files
```sh
# git remote add dotnet https://github.com/dotnet/runtime && git pull --rebase dotnet main

if uname 2>/dev/null | grep -q Darwin; then
    space=" "
fi

git show --name-only --pretty="" HEAD...dotnet/main |\
  xargs -I{} sh -c "test -f {} && sed -i$space'' 's/[[:space:]]*$//' {}"
```

* Enable warn-as-error by default; protect good state