feat: [Grafeas] A new field last_vulnerability_update_time is added to message .grafeas.v1.DiscoveryOccurrence (#8881)

* feat: A new field `last_vulnerability_update_time` is added to message `.grafeas.v1.DiscoveryOccurrence`

PiperOrigin-RevId: 861286340

Source-Link: googleapis/googleapis@70da46f

Source-Link: googleapis/googleapis-gen@dab19f1
Copy-Tag: eyJwIjoiR3JhZmVhcy8uT3dsQm90LnlhbWwiLCJoIjoiZGFiMTlmMTA0NDBkY2QzM2MyYjUyMTAzMGU3ZmU4N2FmZjlmOTgxOSJ9

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]