insights: mark settings oob migration as deprecated as of 3.43

Created by: leonore

closes #38420 (closed)

followed the deprecation step in the oob guide: https://docs.sourcegraph.com/dev/background-information/oobmigrations#step-5-mark-deprecated (which has recently changed)

Test plan

built sourcegraph locally with no errors

(nb. not sure if with the new way of creating oob migrations there's a new way to test them?)