[JENKINS-35551] Upgrade to Credentials v2

[offa]

A first effort of Credentials v2 (#138, JENKINS-35551).

I'm not very familiar with the API, most of it is based on the credentials docs and the linked issues. I'd appreciate someone with experience having a look at it – even more as it's security related.

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• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[offa]

/cc @stephenc

[scaytrase]

thanks @offa !