Update security-tracker data for end of life of regular security support for stretch
Wit the "end of life" of regular security support for Debian stretch and the move to the LTS team some changes are needed so that the respective teams can continue their work.
This merge requests implements the needed changes, following similar approach for the earlier merge request !6 (merged) for the move of Jessie from regular support to LTS support.
Edited by Salvatore Bonaccorso