Add a warning about upcoming legacy system shutdown to the UX

[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 Web Dashboard telling the users and asking them to share with us their usecases in that dashboard.

A message more or less like this:

This legacy KernelCI Web Dashboard will shutdown sometime soon in favor of our new KernelCI system. 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