Cleanup legacy code following `Pythagoras` era activation

[jpraynaud]

Why

We have activated the Pythagoras era and therefore we can clean the legacy code introduced with Thales era.

What

Get rid of the legacy code from Thales era.

How

• Remove optional fields in the messages:
  • EpochSettingsMessage: cardano_transactions_signing_config and next_cardano_transactions_signing_config
  • RegisterSignatureMessage: signed_message
• Remove the deprecated code:
  • PendingCertificate
  • CardanoDbBeaconMessagePart and SignedEntityTypeMessagePart
  • EpochSettingsMessage: protocol_parameters and next_protocol_parameters
• Clean backward compatibility tests in messages that applies to open api version older than 0.1.35 (the version embedded in the first Pythagoras era distribution: 2445.0)
• Squash database migration main store
• Update OpenAPI specs if necessary
• Update documentation if necessary
• Run a backward compatibility test and make sure there is no regression

[Alenar]

Backward compatibility results

Over the 6 last versions: https://github.com/input-output-hk/mithril/actions/runs/13542081368

Signers:

• compatible starting distribution 2445.0 (version 0.2.209)
• incompatible with distribution 2442 (version 0.2.200 , ⚠ 3 signers, 1.77% stake ⚠), cause: removal of protocol/next_protocol in /epoch-settings

Aggregator:

• only compatible with last distribution 2506.0 (version 0.7.1)
• with previous distribution distribution there's a Unsupported era 'thales'. raised on the mithril-signers (I fail to understand why ? E2E runner should only run on the Pythagoras era now)

Client:

• compatible starting distribution 2450.0 (version 0.10.1)
• past versions are incompatible due to the remove of the network field in the CardanoDbBeacon