Complete Deprecation and Removal automation to release post

Why is this change being made?

Closes Product#5157 (closed)

• Renames bin/release-post-deprecations to bin/release-post-deprecations-removals
• Adds ReleasePosts::Removals.new.generate to bin/release-post-deprecations-removals
• Creates lib/release_posts/removals.rb to fetch the Removals list from Docs
  • (unfortunately, the HTML structure is different so lib/release_posts/deprecations.rb cannot be shared)
• Adds deprecations and removals methods to helpers/release_post_helpers.rb
• Updates the release post template to use those methods to get the deprecations and removals from the *-index.yml files for the given milestone
• Updates config[:latest_deprecation_release] from 14.3 to 14.0
  • (to match where the announcements begin in the Docs)
• Adds a job .gitlab-ci.yml
  • (will depend on a schedule with RELEASE_POST_DEPRECATIONS_REMOVALS == true once this MR is approved and merged)

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