[dependabot] Ignore all fixture directories

Seems like we can specify a wildcard dependency name to ignore all
dependencies from being updated. As I understand it dependabot will
still run monthly but no PRs will be generated.

ghstack-source-id: 64b76bd532663cdc4db10ba6299e791b5908d5b1
Pull Request resolved: https://github.com/facebook/react/pull/31074