Add announcement date to release post body

What does this MR do and why?

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-com/www-gitlab-com!90421 (merged)

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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