legal: Clarify "All rights reserved" in sources

Hi,

May I raise an issue I've seen this in many other OSS projects,

IMHO, this copyright mention is not compatible with opensources/free licenses
which is creating an ambiguity in community:

https://mastodon.social/@rzr/108963300576529740#AllRightsReserved

What is preventing copyright or license ?

May I also suggest to add SPDX headers in all project files, something like:

```
# SPDX-FileCopyrightText: Apache Software Foundation
# SPDX-License-Identifier: Apache-2.0
```

This tool can be helpful:

https://reuse.software/

Hope this helps, if not please close this noisy report :)