Update dependabot.yml

[lopopolo]

Bundle dep upgrades into a rollup PR using dependabot groups.

Works around this warning that appears on some Artichoke repositories:

Dependabot updates are paused

We noticed you haven't used Dependabot in a while, so we've paused new pull requests for you. Merge a Dependabot pull request to resume automatically creating Dependabot updates or disable security and version updates. See open Dependabot pull requests

See:

• https://github.blog/changelog/2023-06-30-grouped-version-updates-for-dependabot-public-beta/
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups