use ALTER ROLE for wal_sender_timeout instead of ALTER SYSTEM

[DmitriiAn]

Managed services usually do not allow you to run

ALTER SYSTEM SET wal_sender_timeout = 0;
SELECT pg_reload_conf();

Instead, we can run
ALTER ROLE migration_user SET wal_sender_timeout = 0;

[DmitriiAn]

once approved, we also need to update our docs