Add filters to the Deprecations page

What does this MR do and why?

Adds a filter to the deprecations page to allow filtering by milestone.

Review app http://1172-deprecations-filters.35.193.151.162.nip.io/ee/update/deprecations.html

Screenshots

How to set up and validate locally

• Pull down this branch on the main GitLab repo
• Run the docs site with bundle exec nanoc view
• Try out the filter here: http://localhost:3000/ee/update/deprecations.html

Issue: #1172 (moved)