License file does not match the source file headers

Since d2814d4, all the template source files are licensed under the GPLv3, same as Notepad++. But the content of [license.txt](https://github.com/npp-plugins/plugintemplate/blob/319c80b1161b332a203b7c6df2cadca6d68a9a88/license.txt) hasn't been updated: 

<img alte="gh-gpl-v2" src="https://github.com/user-attachments/assets/db63b264-e082-4dcd-b47e-e2fc88561fdc">
