Add release post labels to planning process

Why is this change being made?

We tend to wait until the end of the milestone to generate release posts. This causes some inefficiencies around re-opening/closing issues that have already been closed to utilize automation, but it also causes a lot of work to have to be done at the last minute to avoid missing the Release Post. As a result, we've recently had several features make it into a release, but not get announced until the next milestone.

This MR proposes that release posts are automatically generated at the beginning of the milestone (when planning is finalized) so that we can review them throughout the milestone, and merge as soon as the feature makes it into the release.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • 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 affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---