Support for older APT clients

[epomatti]

When mirroring this repository into Landscape, I get the following error:

Cannot parse './lists/update-stable_%2F_flat_InRelease': found no signature but does not looks safe to be assumed unsigned, either.

We're running the Landscape 24.04 LTS release with the latest updates.

While I can't quite point out the root cause, I do believe the items below would help support additional APT clients which might use different signature options, including Landscape:

• Support additional release signatures, at least SHA256 along with the existing SHA512, and perhaps others.
• Include the Release.gpg signature file as mentioned in DebianRepository format guildeline.

The guideline has this to say about signature algorithms, although most major repositories support it, such as Kubernetes, which is one the many mirrors we currently maintain.

Clients may not use the MD5Sum and SHA1 fields for security purposes, and must require a SHA256 or a SHA512 field.