I have created a fork of a project that is releases under GPLv3 (As per LICENSE-file, GitHub and README).
However, contributions that are from the community are licensed under MIT.
Now since I want to continue this project as a standalone one, I want to understand how I need to treat the upstream source code in regard to the community-license in that repository.
Am I allowed to strip away the MIT license and treat the full project as GPLv3? To me it is unclear if that license applies to the whole source code, or only the part the original maintainer wrote and the community-contributions are still MIT.
To make matters worse, the upstream-project itself is just a fork of a different project, and that project was MIT originally.
I am fine with using either one, but I want to stick to one license and remove the uncertainty.
So to the final question:
What do I need to do to still adhere to the original licensee-requirements, and which license is the one I have to follow in my own project without breaking the license agreement?