Update code to not raise any FutureWarnings

[patrickleonardy]

Update code to not raise any FutureWarnings

Some code in the Cobra library raises FutureWarnings.

Task Description

Change the code such that those FutureWarnings do not end up generating problem later

FutureWarnings

tests/preprocessing/test_target_encoder.py::TestTargetEncoder::test_target_encoder_transform_new_category_linear_regression_median
/home/patrick/github_projects/work/cobra/tests/preprocessing/test_target_encoder.py:312: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
df_appended = df.append({"variable": "new", "target": 10},

tests/preprocessing/test_target_encoder.py::TestTargetEncoder::test_target_encoder_transform_new_category_binary_classification_median
/home/patrick/github_projects/work/cobra/tests/preprocessing/test_target_encoder.py:338: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
df_appended = df.append({"variable": "new", "target": 1},