`testing-sanchonet` network decommission

[jpraynaud]

Why

The Sanchonet Cardano network will be sunset on February 14th, 2025. This means that the testing-sanchonet Mithril network must be decommissioned.

What

Decommission the testing-sanchonet Mithril network.

How

• Clean the code from Sanchonet reference
• Delete the testing-sanchonet related infrastructure in GCP
• Update the documentation (network configurations)
• Create a dev blog post
• Announce in Discord