Migrate to ESLint Flat Config

To upgrade to ESLint 9, we need to reformat our configuration file here: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/.eslintrc.yml

Here's an example from the GitLab project: gitlab-org/gitlab!170174 (merged)

See also:

• https://eslint.org/docs/latest/use/configure/migration-guide
• https://eslint.org/blog/2022/08/new-config-system-part-2/

Done when

• Config files are in "flat" format
• eslint upgraded to latest 9.x release (may require more changes? TBD)