[Ruby] Update min ruby reqts.

[luke-hill]

🤔 What's changed?

• Update min ruby 2.3 -> 2.5
• Lay groundwork to try improve styles
• Remove a load of redundant 2.1/2.2 ruby helpers

⚡️ What's your motivation?

Tidy up / optimise

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)
• 💥 Breaking change (incompatible changes to the API)

♻️ Anything particular you want feedback on?

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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