Update to tuf 3.1

[jku]

A superficial update is easy (in fact no changes are 100% required) but to avoid ugly changes in future, it might make sense to do this properly (I've got a branch with checked items done already):

• Use verify_delegate() from Root, Targets instead of Metadata
• Use signed_bytes property when the payload bytes are needed
• Use get_verification_result instead of verify_delegate where-ever it's more ergonomic
• Use get_verification_result instead of our own similar code in repositorys _get_signing_status()
• Refactor open_prev(): We're really only interested in the previous version of Signed, not the Metadata...
• refactor signer to use get_verification_result instead of own code

optional:

• (maybe) Use get_verification_result in signer to explain who has signed already
• (maybe) Refactor open() in general (this is more tuf API refactor than just tuf-on-ci internals)