Use a single JSON library

At the moment both Jackson and GSON are configured as dependencies. It would be better if there was only one dependency for processing JSON. Hopeful Jackson will be the chosen one.

https://github.com/in-toto/in-toto-java/blob/73afcb46d73cf63f819f10fc591435cdd5c01dd1/pom.xml#L57-L74

Start with the dependency declarations in pom.xml at the linked lines, then search the project for Jackson and GSON usages to identify what would be affected by removing one library. Done means a single JSON library remains configured and the project still builds and its existing tests pass.

Written by the indexing model from the issue text.