chore(migrations): renumber 021-023 to 023-025 to clear the collision with main

main independently shipped 021_hashtag_counts_respect_bans and
022_client_upload_idempotency. sqlx::migrate! embeds ./migrations and refuses two
files per version, so these three had to move before the branch could merge at all.

Contents are untouched — only the version prefixes change. Renumbering (rather than
renumbering main's) is the safe direction: main is already deployed, so its 021 and
022 are applied in production and their versions are now immutable.

backend/migrations/023_derivative_attempts.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS idx_upload_derivative_backfill;
+ALTER TABLE upload DROP COLUMN IF EXISTS derivative_last_error;
+ALTER TABLE upload DROP COLUMN IF EXISTS derivative_attempts;