Remove `Show (Some a)` and `Eq (Some a)`. Add `Show (Some Era)` and `Eq (Some Era)`.

[carbolymer]

Changelog

- description: |
    `Remove `Show (Some a)` and `Eq (Some a)`. Add `Show (Some Era)` and `Eq (Some Era)`.
# 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

A follow-up from: https://github.com/IntersectMBO/cardano-node/pull/6017#discussion_r1816724383

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