Fix cylc-flow setup.cfg metadata e-mail

[kinow]

Describe the bug

Another one found during our latest release. The e-mail for Cylc Flow metadata is using the cylc@googlegroups.com. That probably needs to be updated as we are now using Discourse. @hjoliver I logged in to Discourse and tried finding an e-mail to the Cylc Support category, with no luck.

Not sure what's the e-mail supposed to be, so will leave this issue for later 👍

p.s.: once we find the correct e-mail, we can update Cylc UIS as well

Release version(s) and/or repository branch(es) affected?

Steps to reproduce the bug

Expected behavior

Screenshots

Additional context

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).