source.json should be a source.yaml, just for consistency

[christoph-jerolimov]

PRs like this:

• Add npm workspace to commit 335037a for backstage 1.35.0 on branch release-1.5 rhdh-plugin-export-overlays#948

creates a source.json (manually formatted for readability) like this:

{
  "repo": "https://github.com/backstage/community-plugins",
  "repo-ref": "335037a9f57eef5a989363cb9779cfaeb8c81f9b",
  "repo-flat": false
}

Since everything in the Backstage world is using YAMLs, and also the PR creates a plugins-list.yaml I would recommend to create a source.yaml as well

repo: "https://github.com/backstage/community-plugins"
repo-ref: "335037a9f57eef5a989363cb9779cfaeb8c81f9b"
repo-flat: false