[syslog-ng] moving debian directory from master to release branch on GitHub

Hello! Debian directory is being moved from master to another branch named release. Thus, the source code is separated from the build. If someone wants to create a Debian package, he/she must checkout that branch first. If someone has any concerns or notes, please share either here or in the PR on GitHub. PR: https://github.com/balabit/syslog-ng/pull/1215 kvch