Add monitoring for CD secrets updates

[timja]

Service(s)

Datadog, GitHub

Summary

ref #4382

see the API at:
https://docs.github.com/en/rest/actions/secrets?apiVersion=2022-11-28#get-a-repository-secret

Monitor that the secret at https://github.com/jenkinsci/jenkins-infra-test-plugin/ is getting updated at least every 3-4 hours or so

Reproduction steps

No response