[4.0.0] mysql_info: users_info filter stop returning plugin_auth_string

[laurent-indermuehle]

Summary

Relates to #629

mysql_info: users_info filter stop returning plugin_auth_string

For now:

• Announce via the changelog
• Release the collection

[laurent-indermuehle]

I see multiple ways of doing this:

1. Remove the output from get_existing_authentication() but this risk to modify behavior of user_add() and user_mod().
2. Filter the key plugin_auth_string from the filter's dictionary output before returning. It safest that way.