guides: apertis_release_process: Update reprepro steps to aptly

Task https://phabricator.apertis.org/T8672

The OBS published has been recently switched from reprepro to aptly. Update any old reprepro reference in the release process guide, so it now uses aptly as the APT publisher.

Note that the apertis_release_process guide is pretty outdated, so this commit focuses only on switching from reprepro to aptly. Further task will acomplish updating the whole release process documentation.

Task https://phabricator.apertis.org/T8676 will achieve the goal of no human being required to log into individual machines, which would have impact on the apertis release process guide as well.