Dependency update and some cleanup

[antoine-de]

this is a bit of a holdall PR, but those are quite small changes and I don't want conflicts between them.

• update all the dependencies (with a tool listed by the ANSSI)

  $ cargo outdated
  All dependencies are up to date, yay!

• rename gtfs_rt_utils to model_update since now this module has only one public method, we name it after this method
• rename routes/datasets -> routes/list_datasets and context to datasets(since there are nocontext` anymore)