bump main branch to v5.16.0-dev
bump main branch to v5.16.0-dev

Since we now have a release-5.15 branch, the v5.15.x series will be cut
from there.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

containers/image,containers/image