gh-1099: Group @dependabot updates together for pre-commit/GHA

[paddyroddy]

Description

Uses multi-ecosystem-groups to group pre-commit and GHA updates together for @dependabot. I thought I'd done this in #1073, but it wasn't working.

Closes: #1099

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[Copilot]

Pull request overview

This PR updates the repository’s Dependabot configuration to group GitHub Actions and pre-commit dependency updates together using multi-ecosystem-groups, addressing #1099 (and correcting the earlier attempt from #1073).

Changes:

• Adds a multi-ecosystem-groups definition (ci-dependencies) to centralize schedule/labels/commit-message configuration.
• Updates the github-actions and pre-commit Dependabot entries to reference the ci-dependencies multi-ecosystem group.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.