Drop deprecated `AFFECTEDVERSIONATTRIBUTION.LAST_SEEN` column

### Current Behavior

The column was deprecated in #6979 and is no longer updated. It was not dropped as part of that change to enable for smooth upgrading from 5.0.x to 5.1.

### Proposed Behavior

Drop the column as it's no longer used, and the risk of code still accessing it is low for users coming from 5.1.x.

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/main/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested

Start by locating the database schema or migration that defines AFFECTEDVERSIONATTRIBUTION.LAST_SEEN, then review the migration history around issue #6979 and the 5.0.x-to-5.1 upgrade path. Remove the unused column without disrupting supported upgrades, and run the repository's database migration or upgrade tests to confirm the schema remains valid.

Written by the indexing model from the issue text.