Debian repo: Fix Release contents (Origin and Label values)

The Release file contains:

Origin: . stable
Label: . stable

These values (. stable) should better identify the repository. They should be something like LemonLDAP-NG.

See Hashicorp's Release for example:

Origin: Artifactory
Label: Artifactory

Or, of course, Debian's own:

Origin: Debian
Label: Debian

I found this while looking how to configure unattended-upgrades to automatically update.