Remove `Pick into` references from the release post process

Why is this change being made?

The Pick into X.Y labels are deprecated and no longer function to pull changes in to patch releases. The release post process includes using these labels to ensure that any release posts and "What's new" posts are included in any future releases of the associated minor version.

This MR removes references to those labels and adds instructions for the new process which involves opening a 2nd MR targeting the X-Y-stable-ee branch.

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
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  • 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
  • The when to get approval handbook section explains the workflow in more detail
• 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.

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