tech:2995:Use kotlinx-serialization instead of Gson
Description
Weekly report will make more use of Json serialization. It the opportunity to move from the Gson library, to the kotlinx-serialization library.
Pro: it's a kotlin library, which handle nullable and default values (Gson is a Java library, and can produce null value on non-nullable fields on deserializations) Con: it requires to annotate the types to serialize and deserialize.
Tests
Check that saved on device Tracker list is parsed correctly.
Issues
https://gitlab.e.foundation/e/os/backlog/-/issues/2995