Copyright for elbe-keyring source package in Sid

Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/

Files: *
Copyright: 2022 Linutronix GmbH 
License: CC0
 On Debian systems, the full text of the Creative Commons Zero 1.0 Universal
 can be found in the file `/usr/share/common-licenses/CC0-1.0'.