v1.0.6: Clarify and simplify copyright and LGPL license

- Update the copyright header in all source files with a simplified version that uses the SPDX license identifier for the LGPL.
- Combining the two copyright holders into a single line.
- Add the `COPYRIGHT.md` file that clarify who’s holding the copyright and the special propietary use exception for one copyright holder.
- Adding the GPL license along with the LGPL addendum.
- No functional changes were made to the source code.