Remove `polars_dataframe.to_pandas()` from tests

[jonathan-eq]

Now that we are moving to using polars rather than pandas for dataframes, we have a mix of it. #12015 changes localensemble to use polars, but the relevant tests are still using pandas, and we have an ad-hoc polars_dataframe.to_pandas().... This should be removed, but it requires rewriting the tests a bit and perhaps update a couple of snapshots.

Tests this includes:

• tests/ert/ui_tests/cli/test_cli.py
• tests/ert/ui_tests/gui/test_csv_export.py
• tests/ert/ui_tests/gui/test_full_manual_update_workflow.py
• tests/ert/unit_tests/storage/test_local_storage.py