Update network versions on README (#3137)

Automated version updates:
  - Fuel Ignition: 0.46.0
  - Testnet: 0.47.1
  - Devnet: 0.47.1

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Update README network versions table: bump Testnet from 0.46.0 to
0.47.1.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
8d6ac0aa10bede526a024206ffee5f209c49e72a. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Mike Christopher <mchristopher@users.noreply.github.com>