This moves the `*TrackerData` models into the `Integrations::` namespace
as part of https://gitlab.com/gitlab-org/gitlab/-/issues/201855.

We also rename the related concerns to clarify their intent:

`app/models/concerns/services/data_fields.rb`:

- Renamed from `Services::DataFields` to `Integrations::BaseDataFields`.
- This is included in the models and acts as a sort of base class.
- It's still a module because converting it into an actual class
  changes some of the automatic Rails behaviour (such as inferring
  the table name).

`app/models/project_services/data_fields.rb`:

- Renamed from `DataFields` to `Integrations::HasDataFields`.
- This is included in the main `Integration` model and provides some
  helpers to access the associated data fields records.

Both of these are moved into `app/models/concerns/integrations`.