Improvements to data validation in CI

Over the past several months, we've identified a handful of specific expectations we have for the data quality in this repo that we've been checking manually during PR reviews. Several of these expectations could instead be asserted by CI. Doing so would reduce toil in advisory PR reviews and increase confidence in the repository's data quality. This issue tracks progress on our short-term data validation goals. **Standard validation** - [x] All known aliases have been captured (for any new/modified documents in the PR) - [x] All referenced packages are currently defined in Melange files in Wolfi ([link to existing issue](https://github.com/wolfi-dev/advisories/issues/387)) - [x] All referenced package versions can be found in Wolfi's APKINDEXes - [x] Advisory `fixed` events **don't** refer to the first published version of a package **Diff validation** - [x] Existing advisory events haven't been modified - [x] Existing advisory events haven't been deleted - [x] New advisory events' timestamps are "recent" (< 3 days old for now)