Prepare new Pythagoras Mithril era

[jpraynaud]

Why

We want to get rid of some legacy code which can only be removed when we activate a new era: in particular the immutable file number reference in the certificate struct and the optional epoch in the signer registration message.

What

Create a new era and prepare the removal of legacy code

How

• Create a new Pythagoras era
• Test era switch in the end to end test
• Enhance the runbook for era markers
• Activate the new era on test networks:
  • testing-preview
  • testing-sanchonet (will be done later)

Later

• Cleanup the Thales era once the Pythagoras is activated
• Activate the new era on test networks:
  • testing-sanchonet
• Prepare a rollout plan for the new era:
  • pre-release-preview
  • release-preprod
  • release-mainnet