Current MIT-License does not suit QGIS Plugin License Requirements

Accoding to the [Licensing requirements for QGIS plugins](https://blog.qgis.org/2016/05/29/licensing-requirements-for-qgis-plugins/), QGIS plugins should comply with GPL version 2 or greater.

Following the discussion in [this QEP](https://github.com/qgis/QGIS-Enhancement-Proposals/issues/279), license files will become mandatory as of June 1st, 2024.

Therefore, we will have to switch to GPL version 2 or greater, and I'd propose using [GPL version 3](https://www.gnu.org/licenses/gpl-3.0.de.html). The current MIT-License is compatible with this, so re-licensing the code is feasible.

This issue exists mainly to inform all our contributors about this change, so they can voice their concerns (if any).
Tagging @nilsnolde @Merydian @TheGreatRefrigerator @mattamach @m-kuhn @jannefleischer @kcrkor for this.