create-testnet-data: use experimental API and make arguments era specific

[smelc]

Changelog

- description: |
    More fine grained controls of eras for create-testnet-data. Forbid creating DReps and CC in babbage.

    Remove the `create-testnet-data` altogether in eras earlier than Babbage
# 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

Fixes #962

How to trust this PR

Look at the changes in golden files

Checklist

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

[palas]

Looks good. I left a couple of comments on things that caught my eye, just minor things. Feel free to address them if you think they make sense, but I leave it to your judgment

[palas]

It doesn't derive anymore? It is odd because Era does have a Show instance (here), maybe after moving the parameter it will?

[smelc]

@palas> it doesn't derive anymore because there's an existential type in GenesisCreateTestnetDataCmdArgs now:

[palas]

Making it unrepresentable on the type would be safer. But this is probably much simpler, so it seems a reasonable trade-off

[palas]

Ah, one more thing, probably say what got broken in the change log. (That some eras no longer have the command.)