Add breaking_change to removals template

Why is this change being made?

Hopefully, we will no longer be using this template to announce removals as part of our effort to introduce a more scalable removals and breaking changes process.

But just in case we don't launch the new process in 14.7, this MR adds a breaking_change key to the removals template that we will use in the future to announce breaking changes and to compile a breaking changes blog post.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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