Draft: Add markup to support filtering by milestone

What does this MR do and why?

Adds HTML markup to deprecations.md to support filtering the page on the docs site by milestone (see gitlab-docs!2622 (merged)).

How to set up and validate locally

Run bin/rake gitlab:docs:compile_deprecations to build deprecations.md.

These changes add markup, but no visible changes on their own.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.