[Mainlining] Update account_validity requests

[giomfo]

During the mainlining of synapse, a new optional module has been introduced to handle the account validity.
This involves a new path for the account_validity requests:
/_matrix/client/unstable/account_validity/* is replaced by /_synapse/client/email_account_validity/*
The request format does not change

For the moment the new module has not been deployed

This path is used client side to request a new email or submit a token (send_mail and renew) - See AccountValidityAPI
I don't think we are using the /renew API

1- Check the occurrences of the AccountValidityAPI use
2- Sync with Ops to set up the account_validity module on DEV platform
3- Update the service to use/test the new path on DEV platform
4- Sync with the Web, Mobile and Ops teams to define a plan to update Tchap clients and the Prod/Preprod infra