Update to new version of frontmatter-check for author list validation

[kjaymiller]

Summary

The local script scripts/check_author_list.py and its corresponding pre-commit hook in .pre-commit-config.yaml were added as a temporary measure to enforce that the author frontmatter field is a list (see PR #815).

This functionality is expected to be added to the upstream frontmatter-check tool.

Acceptance Criteria

• Wait for frontmatter-check to release a version supporting author list validation.
• Update .pre-commit-config.yaml to use the new version of frontmatter-check.
• Configure frontmatter-check to validate the author field.
• Remove scripts/check_author_list.py and the local hook configuration.