Update frontmatter-check to support author list validation

[kjaymiller]

The custom script scripts/check_author_list.py was added in PR #815 as a temporary measure to enforce that the author field in frontmatter is a list.

Once frontmatter-check supports this validation natively, we should:

1. Remove scripts/check_author_list.py.
2. Remove the local hook from .pre-commit-config.yaml.
3. Update .frontmatter_check.yaml to use the new native check.
4. Upgrade frontmatter-check dependency.