Add dependabot
Add dependabot

This has been done so that Dependabot will automatically keep the gem
dependencies updated.

This follows the approach agreed in [RFC 153][1] of avoiding allow lists

[1]: https://github.com/alphagov/govuk-rfcs/blob/main/rfc-153-remove-allowlists-from-dependabot-configs.md

alphagov/gds-api-adapters