Reflect REUSE compliance in LICENSE instead of LGPL 2.1

- Just including the "default" license in the top-level LICENSE file (read by GitHub) is confusing: not everything is licensed under that license and licensing shouldn't be assumed based on it
    - Instead, we follow the REUSE specification v3.0 which means every single file describes its own licensing (or is bulk-licensed with DEP5)

Author: mikkopiu