db/migrations: ensure to drop parentid from posts table (#21493)

db/migrations/postgres/000095_remove_posts_parentid.down.sql

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+-- Intentionally left blank as forward migration is not reversible.

db/migrations/postgres/000095_remove_posts_parentid.up.sql

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+-- While upgrading from 5.x to 6.x with manual queries, there is a chance that this
+-- migration is skipped. In that case, we need to make sure that the column is dropped.
+
+ALTER TABLE posts DROP COLUMN IF EXISTS parentid;