feat: run all migrations in a transaction (#10966)

Updates coder/customers#365

This PR updates our migration framework to run all migrations in a single transaction. This is the same behavior we had in v1 and ensures that failed migrations don't bring the whole deployment down. If a migration fails now, it will automatically be rolled back to the previous version, allowing the deployment to continue functioning.

coderd/database/migrations/000142_proxy_derp.down.sql

@@ -1,5 +1,3 @@
-BEGIN;
-
 -- drop any rows that aren't primary replicas
 DELETE FROM replicas
 	WHERE "primary" = false;
@@ -11,5 +9,3 @@ ALTER TABLE workspace_proxies
 	DROP CONSTRAINT workspace_proxies_region_id_unique,
 	DROP COLUMN region_id,
 	DROP COLUMN derp_enabled;
-
-COMMIT;