Automate clean-up of Attestations

[rjaegers]

Current situation

Currently all generated attestations are kept indefinite, even when the resources that they point to are cleaned up.

Required situation

Extend the current clean up process to automatically clean up attestations. Keeping only the ones that point to active release or edge builds, as is specified in the acceptance criteria below.

Acceptance criteria

• Attestations that don't point to a valid resource are cleaned up
• Attestations from release branches are kept indefinite
• Attestations for open pull requests are kept
• Attestations for the last build on the default branch (tagged as edge) are kept