Update network versions on README

[github-actions]

Automated version updates:

• Fuel Ignition: 0.47.1
• Testnet: 0.47.1
• Devnet: 0.47.3

[cursor]

PR Summary

Low Risk
Documentation-only change updating the stated Devnet version; no runtime or build behavior is affected.

Overview
Updates README.md's network version table to reflect Devnet running 0.47.3 (from 0.47.2).

^{Written by Cursor Bugbot for commit 967bf5b. This will update automatically on new commits. Configure here.}