No longer recommend image signatures

Created by: mbuechse

As shown by @josephineSei in https://input.scs.community/RID5cVHpTaG_AEDYsB9gVQ# the recommendation of image signatures in the image metadata standard is not feasible. As decided in the Team IaaS call on 2023-07-19, the standard should be adjusted, meaning that the paragraph in question be dropped.

Definition of ready:

• It has been decided whether this change requires a new version of the standard (because the current version is stable). ---> NO, (see comment below)

Definition of done:

• A new version of the standard has been created with the paragraph in question dropped.
• The paragraph has been dropped from the standard.
• The test script has been adjusted accordingly (use a command-line option for that). --> unnecessary, for properties have never been checked (being mere recommendations)
• The adjusted script invocation has been incorporated into https://github.com/SovereignCloudStack/standards/blob/issue/267/Tests/testing/scs-compatible-test.yaml
• The new version of the standard has been added to https://github.com/SovereignCloudStack/standards/issues/317