Update git repository layout to follow DEP14 and DEP18
This MR does not change any branch names, only provides a gbp.conf
that follows it.
To implement the branch name changes, you can run:
git branch -m master debian/latest
salsa rename_branch debian/box64 --source-branch=master --dest-branch=debian/latest
salsa update_projects debian/box64 --rename-head --source-branch=master --dest-branch=debian/latest
git branch -m upstream upstream/latest
salsa rename_branch debian/box64 --source-branch=upstream --dest-branch=upstream/latest
(commands from devscripts!427 (merged))