generator discovery_proto_export file uses current copyright year instead of original year

The original copyright year needs to be determined/plumbed through instead of using current copyright year.

https://github.com/googleapis/google-cloud-cpp/blob/d53973077220200e8868071343040ab59fef0762/generator/internal/discovery_proto_export_file.cc#L36

Start in generator/internal/discovery_proto_export_file.cc at the linked line and trace how the copyright year is selected and how generated files receive it. Identify where the original year can be carried through; done means generated output uses that original year instead of the current year, with the relevant generator behavior verified.

Written by the indexing model from the issue text.