Add Convert instance for ConwayEraOnwards to Era

[palas]

Changelog

- description: |
    Added an instance for `Convert ConwayEraOnwards Era`
  type:
  - feature        # introduces a new feature

Context

This PR removed the instance Convert BabbageEraOnwards Era and didn't add an instance for Convert ConwayEraOnwards Era, this makes it very difficult to do the conversion. The current PR address this lack

How to trust this PR

It is very simple and it follows the same pattern as before.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff