RFE: Cleanup orphan EC2 ssh key pairs

[cevich]

When creating images in AWS EC2, packer creates a key-pair (object) to access it. However, if a image-build PR is forced-pushed or build task canceled/aborted packer can't remove it. A new periodic (or add to existing) job should be created to remove old key-pairs from EC2. They're always named packer_<some UUID>.

The danger here is hitting some restriction on the EC2-side for the maximum number of key-pairs. It's also a minor pain to remember to manually clean these things up every now and again (I just removed 22 of them going as far back as December 2023 😢