cardano-testnet: Replace eon-based era witnesses with Exp.IsEra/Era

[Jimbo4350]

Background

cardano-api has introduced a new era API in Cardano.Api.Experimental.Era built around Era era, IsEra era, and EraCommonConstraints. This replaces the older eon-based approach (BabbageEraOnwards, ConwayEraOnwards, babbageEraOnwardsConstraints, etc.), which is now deprecated — see cardano-api #938.

The new API is cleaner: rather than threading specific eon witnesses (ConwayEraOnwardsConway, etc.) through code and calling *EraOnwardsConstraints to bring constraints into scope, callers use Era era / IsEra era and obtainCommonConstraints.

Problem

cardano-testnet tests and infrastructure still use the old eon API extensively. For example, from the new gRPC integration tests in PR #6273:

let ceo = ConwayEraOnwardsConway
    sbe = convert ceo
    ...
babbageEraOnwardsConstraints (convert ceo) $ do
  ...

This should instead use Era ConwayEra / obtainCommonConstraints:

let era = ConwayEra  -- Exp.Era
    sbe = convert era
    ...
obtainCommonConstraints era $ do
  ...

Scope

Audit all of cardano-testnet (src and tests) for usage of:

• *EraOnwards* constructors and witnesses
• *EraOnwardsConstraints functions
• ShelleyBasedEra* era witnesses where Era era suffices

And replace with the experimental era API.