Use release-specific Debian repository URLs for our packages

Instead of using deb.autistici.org/ai3/ we should switch to URLs that include the target Debian release name in them (e.g. /ai3/bullseye). This would allow for a much smoother release upgrade path, where we copy old-stable/* to stable/* and then rebuild packages at our own pace.

• create the new repository structure, for the currently used (buster) release
• set REPOSITORY_NAME in the CI settings for the ai3/ group
• switch float to use the new repo URLs structure in roles/float-base (!219 (merged))
• switch ai3/docker/* repository URLs to the new format as well