processors/github_metadata: use git commit date instead of author date

[nodiscc]

• https://stackoverflow.com/questions/11856983/why-is-git-authordate-different-from-commitdate
• https://git-scm.com/book/en/v2/Git-Basics-Viewing-the-Commit-History
• ref. https://github.com/awesome-selfhosted/awesome-selfhosted-data/issues/332