[Misc] Update license texts and add automated checks for this in CI

Description

This MR fixes all existing license texts and adds REUSE to the CI.

• REUSE requires a license for all files, so this has been added. For the files that can't be commented, I covered them in the REUSE.toml

• The copyright statement at the top of the files has been shortened significantly:

  SPDX-FileCopyrightText: <year-of-creation> CERN
  SPDX-License-Identifier: GPL-3.0-or-later

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #1301