Update frontmatter-check to replace custom author check script

[kjaymiller]

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

This functionality is expected to be available in a future version of frontmatter-check.

Once the new version of frontmatter-check is available, this issue tracks the task to:

1. Update frontmatter-check to the new version.
2. Configure it to enforce the author field as a list.
3. Remove scripts/check_author_list.py and its reference in .pre-commit-config.yaml.