Update tf-text to 2.18.1

[gotadachi]

maybe related: #2267

This pull request updates the embedded TensorFlow Text from version 2.12.1 to 2.18.1.

Currently, TensorFlow Serving container images cannot load models trained using TensorFlow Text 2.16.1 or later.

Sample error:

E0000 00:00:1717728698.868398 1121 mlir_bridge_pass_util.cc:56] Failed to parse __inference___call___16218: Op type not registered 'TFText>RoundRobinTrim' in binary running on (some-models)-5b8c94f664-558vk. Make sure the Op and Kernel are registered in the binary running in this process. Note that if you are loading a saved graph which used ops from tf.contrib (e.g. tf.contrib.resampler), accessing should be done before importing the graph, as contrib ops are lazily registered when the module is first accessed.

The embedded TensorFlow Text version is 2.12.1, which may be missing Op types that were added in versions 2.16 and above. Is there any specific reason for not updating it beyond version 2.12.1?