Automate deprecation additions

Why is this change being made?

Each milestone, we manually create a .yml file with the deprecations for that release.

As we move toward automating the release post, one step we can take is automating the creation of a deprecation index so that the release post simply pulls the list for the matching release from that file.

This MR:

• Adds a Deprecations class to the ReleasePosts module
• Includes a fetch method in the class which scrapes titles from https://docs.gitlab.com/ee/update/deprecations.html
• The titles are grouped by release using nokogiri
• The updated titles are written to data/release_posts/deprecations-index.yml

Follow-up

This MR does not close Product#5157 (closed). The following changes also need to happen:

• Add this script to the pipeline schedule
• Create a similar job to fetch Removals
• Update the release post template to pull from this .yml file instead of the current structure

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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