Same 503 "Error proving withdrawal" across four RPCs

Hey all! Thank you so much for your work on this utility. It's a bit sad to see manual withdrawals fade out of favor for fee-heavy third-party bridges (looking at you, Base).

I'm having difficulty with the prove step. This `503` shows up when I use
- https://eth-mainnet.g.alchemy.com/v2/
- https://mainnet.infura.io/v3/
- https://ethereum.publicnode.com
- https://eth.llamarpc.com

They all go like:

```
❯ withdrawer \
--network base-mainnet \
--withdrawal (redacted) \
--rpc "https://eth-mainnet.g.alchemy.com/v2/(redacted)" \
--private-key (redacted) \
--fault-proofs

INFO [02-13|11:27:31.320] Using RPC suggested gas price suggested-gas-price=109972122
INFO [02-13|11:27:31.362] RPC suggested gas tip cap suggested-tip-cap=99999
CRIT [02-13|11:27:37.753] Error proving withdrawal error="503 Service Unavailable: {\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32011,\"message\":\"no backend is currently healthy to serve traffic\"},\"id\":6}\n"
```

Am I missing something about a suitable RPC provider that expects this method? Or have any recent network upgrades changed/broken the flow?

Thanks in advance for the help!