Update Deprecations instructions on RP page to require announcement date

Why is this change being made?

As per: https://gitlab.slack.com/archives/C3TRESYPJ/p1631784859101900 we want to add the announcement_date into the post body as per the suggested layout:

15.0

xxxx/xx/xx

....body.....

Announced: xxxx/xx/xx

Associated handbook update: gitlab-org/gitlab!70527 (merged)

Related to Product#2864 (closed) and !90623 (merged)

Author Checklist

• Provided a concise title for the 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 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 in 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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