Add checkpoint artifact path prefix to MLflow logger (#20538)

* Add checkpoint artifact path prefix to MLflow logger

Add a new `checkpoint_artifact_path_prefix` parameter to the MLflow logger.

* Modify `src/lightning/pytorch/loggers/mlflow.py` to include the new parameter in the `MLFlowLogger` class constructor and use it in the `after_save_checkpoint` method.
* Update the documentation in `docs/source-pytorch/visualize/loggers.rst` to include the new `checkpoint_artifact_path_prefix` parameter.
* Add a new test in `tests/tests_pytorch/loggers/test_mlflow.py` to verify the functionality of the `checkpoint_artifact_path_prefix` parameter and ensure it is used in the artifact path.

* Add CHANGELOG
* Fix MLflow logger test for `checkpoint_path_prefix`
* Update stale documentation


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Co-authored-by: Luca Antiga <[email protected]>