Replace go-migrate with morph (#19116)

db/migrations/postgres/000051_create_msg_root_count.down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE channelmembers DROP COLUMN IF EXISTS msgcountroot;
+ALTER TABLE channelmembers DROP COLUMN IF EXISTS mentioncountroot;
+ALTER TABLE channels DROP COLUMN IF EXISTS totalmsgcountroot;