Change default era value from Babbage to Conway

[nbacquey]

Changelog

- description: |
    Change default era value from `babbage` to `conway`
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Since most Babbage commands have been decommissioned, it makes no sense for babbage to be the default era, when none is provided to the CLI. This PR changes the default to conway instead. Additionally, it fixes an upstream bug in cardano-testnet where the default era was passed back as an input to cardano-cli.

How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff