Change Deliverable and Stretch labels to candidate:: scoped labels

Why is this change being made?

An issue is either a Deliverable or a Stretch goal, can’t be both. Leveraging the recently introduced candidate:: tags, I recommend using candidate::deliverable and candidate:stretch.

Currently, I’m experimenting with issue boards for planning for a given milestone, and having 3 columns (UX, Deliverable, Stretch) would be great. Unfortunately, today, an issue might be both Deliverable and Stretch (and UX, but this is not a problem, as we usually have a distinct issue for UX).

Example board: https://gitlab.com/groups/gitlab-org/-/boards/2094076?scope=all&utf8=%E2%9C%93&label_name[]=group%3A%3Aconfigure&milestone_title=13.6

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.