Seems to be related to DEV: Re-add polymorphic bookmark columns (#16261) · discourse/discourse@8040b95 · GitHub
This commit is a redo of 2f1ddadff7dd47f824070c8a3f633f00a27aacde
which we reverted because it blew up an internal CI check. I looked
into it, and it happened because the old migration to add the bookmark
columns still existed, and those columns were dropped in a post migrate,
so the two migrations to add the columns were conflicting before
the post migrate was run.