docs: replace dead ethgasstation.info link

[Flotapponnier]

Closes #199.

Summary

The Defender Relayers gas pricing callout links to EthGasStation (https://ethgasstation.info/), a service that shut down around 2022. The domain now returns HTTP 403 from a Cloudflare-parked page:

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

The same sentence links to GasNow (https://www.gasnow.org/), discontinued in 2021; that domain now serves an unrelated WordPress site.

Change

One line in content/defender/module/relayers.mdx:

• removed the EthGasStation and GasNow entries from the list of gas price sources (both services no longer exist)
• kept EtherChain, Blocknative, and Etherscan unchanged
• added a closing 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), as an independent resource for readers evaluating gas oracles

To be transparent: I did not attempt to guess which oracles Defender queries internally today. If the actual source list has changed, happy to update the wording accordingly. No other changes.

[netlify]

✅ Deploy Preview for openzeppelin-docs-v2 ready!

Name	Link
🔨 Latest commit	23ef78b
🔍 Latest deploy log	https://app.netlify.com/projects/openzeppelin-docs-v2/deploys/6a490f06a90315000813e602
😎 Deploy Preview	https://deploy-preview-200--openzeppelin-docs-v2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.