insights: update historical enqueuer to convert jit insights
Created by: chwarwick
Adds logic to the historical enqueuer to find and update existing just in time insights and backfill them. This should only be 1 time logic since new scoped insights will be have their backfills queued at creation time.
resolves https://github.com/sourcegraph/sourcegraph/issues/35817
Test plan
Update existing unit tests
Ran backfiller with code_insights_deprecate_jit
in the following states TRUE|FALSE|NULL
ensured that only scoped insights search based insights were updated.