codeintel: fix oldest stale commitgraph calculation

Created by: Strum355

Previously, the calculation for the oldest stale commit graph used lsif_dirty_repositories.updated_at, which is the last time the commit graph was updated as opposed to when the commit graph became dirty, the former could be a very long time ago when a new upload increments the dirty token, hence the spikes in the graph.

Test plan

manually by EYE 👁