project: Replace original_year and final_year with years

project: Replace original_year and final_year with years

Rather than automatically bumping the final year to the current year,
only bump to the last year for which a file was changed.  I've kept
the 'first-last' syntax, but

  update_copyright.utils.copyright_string

is not the only place you have to edit if you prefer to list them all
explicitly:

  Copyright (C) year-1, year-2, ..., year-n A. U. Thor ...