Store SPDX license list version in .LICENSE files

## Short Description

It would be good to store the SPDX license list version stored with the licenses that are coming from SPDX in the .LICENSE files.

## Possible Labels

- new feature

## Select Category

- [X] Enhancement
- [ ] Add License/Copyright
- [ ] Scan Feature
- [X] Packaging
- [ ] Documentation
- [ ] Expand Support
- [ ] Other

## **Describe the Update**

Currently .LICENSE files for licenses coming from SPDX lack the SPDX license list version. Only when scancode is run, or the scancode source code is inspected, it becomes clear which SPDX license list version was used. This is a very minor addition to the data, but which would make it clearer to see if there were any changes, if there are omissions, and so on. By just looking at the .LICENSE files it is absolutely not clear.

At the time of filing this issue the license list is `3.28` but it appears that according to #4775 it might not have been fully integrated yet.

**How This Feature will help you/your organization**

Transparency, traceability, provenance, all good stuff.

## **Possible Solution/Implementation Details**

Add another attribute `spdx_license_list` to the YAML frontmatter for .LICENSE files for which it is relevant.

## **Example/Links if Any**

## **Can you help with this Feature**