Add breaking changes blog epic template

What does this MR do and why?

CLOSED AND MOVED TO gitlab-com/www-gitlab-com!94807 (closed)

Merge DUE DATE: December 10, 2021

We need a template to start driving breaking changes blog start in 14.7.

Related to: gitlab-com/Product#2785 (closed)

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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