CONTRIBUTING: don't mention "ready-for-review"

Phasing out the "ready-for-review" label in favor of GitHub's "Draft
Pull Request" system, per
#821.

Also removed the mention of the "work in progress" label; I think we
want to use this for PRs that need significant redesign rather than PRs
where the author is requesting additional help.

CONTRIBUTING.md

@@ -57,12 +57,9 @@ If you need any help when contributing to the project, reach out to us either of
 Once you have a contribution ready, complete the following steps to submit the contribution:
 
 1. Submit a pull request (PR).
- 1. Set the necessary [labels](https://github.com/rpm-software-management/dnf5/labels).
- * If you need help while working on the PR, set the
- [_work in progress_](https://github.com/rpm-software-management/dnf5/labels/work%20in%20progress) label
- and wait for the PR to be triaged. Triaging usually happens on Mondays or Thursdays.
- * If you are ready for a code review by one of the maintainers, set the
- [_ready for review_](https://github.com/rpm-software-management/dnf5/labels/ready%20for%20review) label.
+ * If the PR is not yet ready to be reviewed by the maintainers, create a
+ "Draft Pull Request". Then, when it's ready, mark it as "Ready for
+ review".
 2. Review the PR.
  1. Discuss the PR with one of the maintainers. Note that the maintainers might request some changes.
  2. Review the PR again.