Update MT5Config (`is_gated_act`)
# What does this PR do?

Update MT5Config (`is_gated_act`). Fix #17578. (Just copy-paste from `T5Config`)
_The documentation is not available anymore as the PR was closed or merged._<|||||>@patrickvonplaten You forgot to edit the title during merge and it became `fix` in the commit page. No worry, just to be a bit bit slower next time 🙏  Thank you :-)