Update instructions for RP extras and upgrades

To suggest a change to the Product Handbook, please review the updating the Product Handbook section.

Related to:

Please provide a brief explanation for this change:

The extras and upgrades type release post items do require the use of a specific template for the MR. There are no recommended labels for these release post items either. This makes it difficult for the release post manager to keep track of these types of release post items.

This MR proposes add the following label requirements to these types of release post items.

• extras: release post, release post item, release post item::extras
• upgrades: release post, release post item, release post item::upgrades

The MR also adds the requirement for both release post item types to add the Ready label after the TW review is complete.

The labels release post item::extras and release post item::upgrades do not currently exist. Once the MR is merge we will need to create these new labels.

These labels will mean that the release post manager can applied filter targeting the milestone and release post item label to see a list of all release post items including extras and upgrades to track for a particular release post.

Please indicate the types of revisions being suggested for the Product Handbook (please check all that apply):

• Small improvement (typos, clarifications, etc.)
• Adding a new section
• Modifying existing section
• Documenting a new process
• Adding a new page or directory
• Other

Please indicate Milestone

• Assigned to Milestone

Author Checklist

• Provided a concise title for the MR
• Provided a brief explanation for this change (Say why, not just what)
  • (Attach screenshots, Slack conversations, etc. as necessary)
• Indicated the types of changes included in this MR
• Verified that no confidential data is in this MR
• Assigned reviewers for this change to the correct DRI(s)
  • 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 #product or #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.