Legal API - Update to use new models module (DO THIS LAST)

After https://github.com/bcgov/entity/issues/23124 is done, the new models module can now be used with all BE components.

Since the models have been moved from Legal API to a common separate one, we want Legal API to use those.

Need to handle:
- [ ] update requirements.txt to reference models module
- [ ] update code references to refer to new models module
- [ ] remove old models from legal api
- [ ] any other required updates
- [ ] fix unit tests

First confirm that issue #23124 is complete, then inspect requirements.txt, the Legal API model references, and the unit tests. The work is done when Legal API references the separate models module, its old models are removed, requirements and other affected references are updated, and the unit tests pass.

Written by the indexing model from the issue text.