Use SPDX for all copyright headers

[KyleFromNVIDIA]

Issue: https://github.com/rapidsai/build-infra/issues/297

[pentschev]

There are some files that have the wrong (Apache 2.0) license, I left requests for changes. The project is BSD-3-Clause unlike most other RAPIDS projects because we want a license matching that of the UCX project.

There are also a few files, namely RAPIDS.cmake, rapids_config.cmake and get_rmm.cmake, that are Apache 2.0 license because they're copies from other projects, can we relicense them to BSD-3-Clause here? This doesn't have necessarily to be done in this PR, but it's a question we should answer because when we upstream UCXX to the UCX repository, then we may need to remove/rewrite those files if the Apache 2.0 license is incompatible.

[KyleFromNVIDIA]

I left the files from other projects as Apache-2.0 because I don't know if they've received external contributions that would make us unable to relicense them. I'll have to do some digging to find that out.

I'll relicense the ones that were already part of this project.

[KyleFromNVIDIA]

I wrote _version.py myself, so I know that one is fine to relicense. Still investigating the others.

[KyleFromNVIDIA]

I've been informed by @robertmaynard that all of the files are safe to relicense.

[pentschev]

Great, thanks so much Kyle!

[KyleFromNVIDIA]

/merge