bert: properly mention deprecation of TF2 conversion script
Hi,

after training some BERT models with the TF2 implementation from the TF models repo, I just made the following observations:

The training code was deprecated a while ago, and now lives under the `legacy` namespace. I have trained models with this "legacy" implementation but the TF2 conversion script is no longer working.

I did some debugging, trained a few models and found out that the latest working version is v2.3.

For that reason, I have just updated the description in the TF2 conversion script.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @LysandreJik , unfortunately, there's no working example code for pre-training in the TF models repo yet (I did some experiments, but I was not able to get a working setup). However, there's a Roformer code (https://github.com/tensorflow/models/tree/master/official/projects/roformer) available and training is working.<|||||>Sounds good, let's merge the PR like this then. Thanks for your contribution!