Prepare new `Lagrange` Mithril era for SNARK support

[jpraynaud]

Why

In order to support SNARK proofs, some changes must be implemented in the certificate chain. This requires to have all the Mithril signers aligned to avoid breaking the certification process and thus necessitates the creation of a new Mithril era.

What

Create a new Lagrange Mithril era.

How

• Create a new Lagrange era
• Prepare removal of legacy code
• Test era switch in the end to end test
• Activate the new era on dev networks:
  • dev-preview
  • dev-follower-preview