Deprecate month host report emails

[Betree]

We want to drop monthly host report emails in favor of the platform reports: https://github.com/opencollective/opencollective/issues/7215. We believe that this report provides better accuracy and legibility than the email one. It is also easier for us to maintain.

### Tasks
- [ ] Remove feature flag for new report
- [ ] Move legacy report to something like `reports-legacy`
- [ ] Add a big warning message at the top of the host report email to say that we will stop sending them in 2025, with a link to the dashboard and a contact email for them to let us know if they see major issues.
- [ ] 
- [ ] After January 2025, do not send the email
- [ ] Remove all related code from the API
- [ ] Close https://github.com/opencollective/opencollective/issues/6232

[znarf]

Let's make sure to send the email for the last time in January (December report). I also suggest a grace period before deleting the code, maybe some people will still want it and we may be asked to send some manually.

[Betree]

All done