Deprecate gelu

[WindQAQ]

Description

Brief Description of the PR:

tensorflow/tensorflow#41178 got merged today (7/27). I suppose it won't appear in TensorFlow 2.3.* unless it's cherry-picked by someone.
@seanpmorgan Can we do that or ask someone to do that or just wait for next release? Thank you.

Fixes #2005.

Type of change

• Bug fix
• New Tutorial
• Updated or additional documentation
• Additional Testing
• New Activation and the changes conform to the activation contribution guidelines
• New Callback and the changes conform to the callback contribution guidelines
• New Image addition and the changes conform to the image op contribution guidelines
• New Layer and the changes conform to the layer contribution guidelines
• New Loss and the changes conform to the loss contribution guidelines
• New Metric and the changes conform to the metric contribution guidelines
• New Optimizer and the changes conform to the optimizer contribution guidelines
• New RNN Cell and the changes conform to the rnn contribution guidelines
• New Seq2seq addition and the changes conform to the seq2seq contribution guidelines
• New Text addition and the changes conform to the text op contribution guidelines

Checklist:

• I've properly formatted my code according to the guidelines
  • By running Black + Flake8
  • By running pre-commit hooks
• This PR addresses an already submitted issue for TensorFlow Addons
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• This PR contains modifications to C++ custom-ops

How Has This Been Tested?

If you're adding a bugfix or new feature please describe the tests that you ran to verify your changes:

• Just add deprecation warnings

[bot-of-gabrieldemarmiesse]

@AakashKumarNain

You are owner of some files modified in this pull request.
Would you kindly review the changes whenever you have the time to?
Thank you very much.

[bhack]

Finally! Just a question: Who is in charge of https://github.com/tensorflow/community/pull/252/files#diff-735bb6a263b2ce7cce140d4b44367e9cR65-R68?

[bhack]

In the meantime tensorflow/models#8977

[bhack]

Are we going to deprecate also GELU codeowners?

[AakashKumarNain]

I am wondering how will the contribution be counted in this case.