Add a warning to the email reports telling users that legacy system will shutdown

[padovan]

We starting to plan the shutdown of the legacy system (no exact dates yet), so we want to have a warning in the current legacy email report telling the users and asking them to share with us their usecases if they are still using the reports

A message more or less like this:

This legacy KernelCI providing this report will shutdown sometime soon in favor of our new KernelCI system. Not all tests are being migrated. If you are still using it and want us to prioritize your usecase in the new system, please let us know at kernelci@lists.linux.dev

[padovan]

Completed. And we already got one message in the mailing list.