bts: limit stderr logging to NOTICE to avoid long write() to ext4 fs
stderr being redirected to a file in the ext4 filesystem sometimes ends up in write() syscall taking >500ms, which means the entire osmo-bts process stalls during that time and we miss clock updates up to a threshold which makes osmo-bts process exit with an error.
Let's decrease logging verbosity to NOTICE for now with the aim to not run into those stalls during normal operation, while we can re-eanble this manually when debugging the write() issue.
