Fix JENKINS-61199: enforce retention for deleted jobs

[ch8matt]

Testing done

Fixes JENKINS-61199: retention by age wasn’t applied to deleted jobs.
JobConfigHistoryPurger.purgeHistoryByAge() only purged getJobs(""), which excludes *_deleted_* roots.
This PR adds a purge pass over deleted jobs via getDeletedJobs(""), without changing behavior for normal jobs.

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[StefanSpieker]

Thanks a lot @ch8matt - I will take a look over the weekend. On a first look, it seems right 👍

[ch8matt]

Hey, Any thoughts on this? :)