Simplify release post manager tasks

Why is this change being made?

This MR simplifies the release post MR description to remove some tasks that are no longer necessary and rewords some of the tasks for brevity and clarity.

• Remove Reminding and Alerting DRIs section. The Slackbot will take this responsibility as well as the direction for allowing PMs to hold themselves accountable for this work.
• Remove you'll do various Slack reminders/announcements note
• Remove redundant label checks
• Reorg opening tasks to be grouped by calendar, -prep channel, and release-post channel
• Provide a template for intro messages
• Remove Recurring content blocks section because "As release post manager, you do not need to act on the checkboxes below"
• Rewrite the exec review section for brevity and provide a template
• Remove tasks that are covered by the Slackbot

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, and the content is SAFE
• 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
  • 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.

---