Replace go-migrate with morph (#19116)

db/migrations/postgres/000036_create_sharedchannelusers.down.sql

@@ -0,0 +1,5 @@
+CREATE INDEX IF NOT EXISTS idx_sharedchannelusers_user_id ON sharedchannelusers (userid);
+
+ALTER TABLE sharedchannelusers DROP COLUMN IF EXISTS channelid;
+
+DROP TABLE IF EXISTS sharedchannelusers;