fix: avoid copy argument warning in to_pandas

[tswast]

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Fixes internal issue b/508671576 🦕

[gemini-code-assist]

Code Review

This pull request removes the deprecated copy parameter from set_axis calls in blocks.py and dataframe.py. Feedback suggests that instead of using set_axis, which may still perform a copy in newer Pandas versions, the code should directly assign to the .columns attribute to ensure efficiency and avoid deprecation warnings.

[parthea]

Switching to draft until presubmits are green

[tswast]

Failed tests:

FAILED tests/system/small/bigquery/test_ai.py::test_ai_if_multi_model - asser...
FAILED tests/system/small/bigquery/test_ai.py::test_ai_classify_multi_model

Appear to be unrelated to this change.