Update network versions on README

[github-actions]

Automated version updates:

• Fuel Ignition: 0.47.4
• Testnet: 0.48.0
• Devnet: 0.48.0

[cursor]

PR Summary

Low Risk
Documentation-only change that updates displayed version numbers; no impact on binaries, APIs, or behavior.

Overview
Updates the README.md network version table to reflect current deployments: Fuel Ignition to 0.47.4, and Testnet/Devnet to 0.48.0. No code or runtime behavior changes.

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