Update default user table migration

[timptner]

Change default admin email to a valid domain for testing purposes provided by ICANN.

[timptner]

Solves issue #3946.

[ssddanbrown]

Thanks for offering this change @timptner but I'm going to close this off without merge.
Change of the default credentials will cause a lot of existing documentation & processes to become incorrect/outdated & in need of update, It would effectively be a breaking change in my view that requires some co-ordination and warning.

I do agree it would be good to move away from the existing credentials, but I don't think the reasons in #3946 are worth it alone. If we are to make changes to the default credential process, I'd like to do so as part of more substantial changes (An initial setup process or randomly generated default creds or something similar, needs some thought).