Update `mlflow` image

### Why it needs to get done

The `mlflow` component of `dss` gets deployed using `ubuntu/mlflow:2.1.1_1.0-22.04`, but we already have a rock based image (hosted at [canonical/base-mlflow](https://github.com/canonical/base-mlflow)). The goal of this task is to use the latest container image created from the rockcraft project.

### What needs to get done

Replace the current value of [`image`](https://github.com/canonical/data-science-stack/blob/main/poc/common-manifests/mlflow.yaml#L36C18-L36C47) with `charmedkubeflow/mlflow:v2.15.1-d756ce7`

### When is the task considered done

The latest version of `dss` uses `charmedkubeflow/mlflow:v2.15.1-d756ce7`.