Draft: Update deprecations YAML template

What does this MR do and why?

Proposes updates to the deprecations' YAML template:

• Same as we have for release post items, like reporter.
• A couple more matching the info already present (like a link to the deprecation announcement blog post if it exists).
• Call out that the URLs should be full rather than relative.
• Add periods to the end of all sentences (🤓).

Ref: https://about.gitlab.com/handbook/marketing/blog/release-posts/#deprecations

How to set up and validate locally

I think these updates don't really change how the deprecations will render.

MR acceptance checklist

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