pandas change append -> concat (append in dfs removed in pandas>=2.)

[skon7]

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resolves https://github.com/keras-team/keras-io/issues/1518 and https://github.com/keras-team/keras-io/issues/1526

Issue:

https://github.com/keras-team/keras-io/issues/1518 (AttributeError: 'DataFrame' object has no attribute 'append')

Description

This pull request addresses issues https://github.com/keras-team/keras-io/issues/1518 and https://github.com/keras-team/keras-io/issues/1526 by replacing the append function with the concat function. The issue was caused by a compatibility problem with pandas versions greater than 2.