Remove ui_color/ui_fgcolor from SerializedBaseOperator

Split from PR #60435.

`airflow-core/src/airflow/serialization/definitions/baseoperator.py` has TODO: "Remove the following, they aren't used anymore" for `ui_color`/`ui_fgcolor`.

Decide on removing these fields from the serialized model and schema, and verify no downstream consumers depend on them.

Start in airflow-core/src/airflow/serialization/definitions/baseoperator.py and inspect the TODO for ui_color and ui_fgcolor. Trace the serialized model and schema references, then check whether downstream consumers still depend on these fields. Done means removing them only where safe and verifying that no downstream consumer relies on them.

Written by the indexing model from the issue text.