Validation error: Low difficulty: 156086
While running `xmrig 6.15.0` against `monero-pool` build on Sep 11, I get this:

`stagenet`

```sh
08:24:33 INFO  src/pool.c:3830: New miner [127.0.0.1:57270] connected
08:24:33 INFO  src/pool.c:3834: Pool accounts: 0, workers: 1, hashrate: 0
08:24:33 TRACE src/pool.c:3034: Miner login for mode: self-select
08:24:33 TRACE src/pool.c:3047: Miner set rigid: GHOST-M1
08:24:33 DEBUG src/pool.c:1161: Miner d63e01b4a3af49ad8a41ed357e256e16 target now: 1000
08:24:33 TRACE src/pool.c:1399: Miner job: {"id":1,"jsonrpc":"2.0","error":null,"result":{"id":"d63e01b4a3af49ad8a41ed357e256e16","job":{"job_id":"2fabc4d935494338b97c5d0cff0dbf65","target":"37894100","extra_nonce":"01000000ad8a9719", "pool_wallet":"55V8qFmyuNQ2fHpYBNzi8UdAh1TfZUyw7125jLR7TWsZDuZHGAa7PDGFJoApMA9C1gPMMjxmj4PdLLZ842kY9d6TJCqf9Mv","seed_hash":"","next_seed_hash":""},"status":"OK"}}

08:24:33 DEBUG src/pool.c:1357: [127.0.0.1:57270] Validation error: Low difficulty: 156086
08:24:39 DEBUG src/pool.c:1357: [127.0.0.1:57270] Validation error: Low difficulty: 156107
08:24:45 DEBUG src/pool.c:1357: [127.0.0.1:57270] Validation error: Low difficulty: 156107
08:24:50 DEBUG src/pool.c:1357: [127.0.0.1:57270] Validation error: Low difficulty: 156107
08:24:55 DEBUG src/pool.c:1357: [127.0.0.1:57270] Validation error: Low difficulty: 156107
08:25:01 DEBUG src/pool.c:1357: [127.0.0.1:57270] Validation error: Low difficulty: 156107
```
And that just keep going until restart.
