docs(licensing): clarify core library license locations

Update the main LICENSE file to reflect that copies of the core library's LGPL-3.0 license can now be found in both `src/rotator_library/COPYING` and `src/rotator_library/COPYING.LESSER` files, instead of a single `LICENSE` file.

LICENSE

@@ -1,6 +1,6 @@
 This project contains components with different licenses.
 
-- The core library, located in `src/rotator_library/`, is licensed under the GNU Lesser General Public License, Version 3.0. A copy of the license can be found in `src/rotator_library/LICENSE`.
+- The core library, located in `src/rotator_library/`, is licensed under the GNU Lesser General Public License, Version 3.0. Copies of the license can be found in `src/rotator_library/COPYING` and `src/rotator_library/COPYING.LESSER`.
 
 - The proxy application, located in `src/proxy_app/`, is licensed under the MIT License. A copy of the license can be found in `src/proxy_app/LICENSE`.