Update Flax removal version

[DN6]

What does this PR do?

Bump up slated removal of Flax related code to 0.40.0

Fixes # (issue)

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[yiyixuxu]

thanks

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[sayakpaul]

Failing tests are unrelated.