Enable GitHub Actions Updates in Renovate

[harryswift01]

Summary

Updates the custom Renovate configuration to include GitHub Actions dependencies alongside the existing Python dependency management.

Changes

Enable GitHub Actions Manager

• Added github-actions to the enabledManagers configuration.
• Retained the existing pep621 manager for Python dependencies.

Impact

• Allows Renovate to detect and propose updates for actions used in GitHub Actions workflows.
• Expands automated dependency management beyond Python dependencies without changing any existing Renovate behaviour.