Consolidate slack messages for PMs in release post MR template

Why is this change being made?

Some days during the release post cycle include multiple updates to all PMs in the #release-post channel. This resulted in either multiple pings for the entire PM team or a post without mentioning the PMs that many might miss. This is to consolidate reminders into single Slack messages that mention the PM team, so nothing is missed and they don't receive more mentions than necessary.

Relates to #13922 (comment 1109428139)

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.

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