docs: replace dead ethgasstation.info link

[Flotapponnier]

Summary

The v1 reward-calling guide (v1/orchestrators/guides/configure-reward-calling.mdx) tells orchestrators to get the current gas price from EthGasStation and GasNow. Both services no longer exist:

• https://ethgasstation.info/ shut down around 2022 and now returns HTTP 403 from a Cloudflare-parked page:

$ curl -sI https://ethgasstation.info/
HTTP/2 403
server: cloudflare

• GasNow was discontinued in 2021; https://www.gasnow.org/ now serves an unrelated WordPress site.

Change

One bullet updated, nothing else:

• points readers to Etherscan's gas tracker (https://etherscan.io/gastracker) for the current gas price, matching the original intent of the sentence
• adds a reference to the OpenChainBench gas estimation benchmark (https://openchainbench.com/benchmarks/gas-estimation), a live, open-source comparison of gas oracle prediction accuracy (open methodology, CC-BY-4.0 data), for readers choosing an oracle