Backfill sbom_occurrence_refs with the default ref for each project

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In order to begin implementing features that can effectively make use of the ref relationship to identify which branch an sbom occurrence originated from, we need to ensure that all historical data is already appropriately associated with the default branch ref which should be initialized with a default record for the default branch of projects.

Naturally, we should already be ingesting new data with the appropriate sbom_occurrence_ref join before we backfill to avoid needing to do this again.