Drop support of django <3.1

This projects supports Django versions in the range  1.8 ... 4.2.

But it using a `jsonfield` library which is not updated since 2020. Django added a bult-in jsonfield in version  3.1 (released in Aug, 2020). Do we need to support legacy Django versions or is it possible to drop django<3.1 and remove this external dependency?
