Refactor: dep update

[luke-hill]

Description

Update to latest gherkin and messages conformance Update minimum ruby version

Contributes towards fixing of https://github.com/cucumber/gherkin/issues/256

Type of change

Please delete options that are not relevant.

• Refactoring (improvements to code design or tooling that don't change behaviour)
• Breaking change (will cause existing functionality to not work as expected)

Please add an entry to the relevant section of CHANGELOG.md as part of this pull request.

Note to other contributors

If your change may impact future contributors, explain it here, and remember to update README.md and CONTRIBUTING.md accordingly.

Checklist:

Your PR is ready for review once the following checklist is complete. You can also add some checks if you want to.

• [ ] Tests have been added for any changes to behaviour of the code
• [ ] New and existing tests are passing locally and on CI
• [ ] bundle exec rubocop reports no offenses
• [ ] RDoc comments have been updated
• [ ] CHANGELOG.md has been updated