Refactor Release Post Deprecation and Removal Indexers

Why is this change being made?

Background

Specially formatted YAML files in https://gitlab.com/gitlab-org/gitlab/-/tree/master/data/deprecations?ref_type=heads are our single source of truth for GitLab deprecations. They are used to build this page in the docs: https://docs.gitlab.com/ee/update/deprecations.html

For Release Post, we introduced the titles of those same deprecations into the Release Post itself. We did so through routinely screen scraping the deprecations page HTML source which was used to build out an index of those deprecations in a YAML file, https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/data/release_posts/deprecations-index.yml However, the screen scraping approach is fragile.

Problem

• The current screen scraping approach is fragile requiring fixes and tight coupling to the design of the deprecations documentation page.
• We should use the SSOT yaml files to build the deprecations and removals lists on the release post.

Approach

We also generate an RSS feed of breaking changes from the deprecation YAML files. The refactoring approach here will pull deprecation files to create the indexes for deprecations and removals in a similar manner to the RSS generator.

This iteration intentionally duplicates logic across Deprecations and Removals. It would be more performant make a single set of API calls and parse several YAML files only once, but at this stage I would prefer duplication to the wrong abstraction. In a future iteration, we can consider other refactoring this again for performance rather than results.

Related issue #34291 (closed)

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