Replace the `TimonVS/pr-labeler-action` with
`release-drafter/release-drafter` as it has an [`autolabeler`][1]
option that can autolabel PRs for us.
This should fix labeling PRs from forks,
see https://github.com/TimonVS/pr-labeler-action/issues/25.
I've kept the `.github/pr-labeler.yml` configuration file, so that
links to it from the https://mermaid.js.org website continue to work.
I've also kept everything in the same
`.github/workflows/pr-labeler.yml` GitHub Actions workflow to make the
`git diff` easier to review, and to keep the GitHub Actions permissions
the same.
[1]: ff929b5ceb/README.md (autolabeler)
