Use deprecations page to list the removals as well

Why is this change being made?

The deprecations and removals pages have duplicate content and this is confusing to the users.

There's a discussion to use only the deprecations page as the SSoT, This MR does that.

Specifically:

1. Copy the logic of deprecations.rb to removals.rb.
2. Change how deprecations are parsed (search for Announced in: GitLab occurrences).

Currently, the way the deprecations are constructed is wrong. For example, if you go to the 16.0 deprecations and click on an item, for example https://docs.gitlab.com/ee/update/deprecations.html#cas-omniauth-provider, you'll see that this was announced in 15.3, but it's listed as a deprecation in the 16.0 release post.

Here's how it's in the review app https://axil-update-deprecations-removals-libs.about.gitlab-review.app/releases/2023/05/22/gitlab-16-0-released/#release-deprecations

How to test

In https://docs.gitlab.com/ee/update/deprecations.html:

• The number of a release's deprecations are the items when you search the string Announced in: GitLab <ver>, where <ver> the GitLab version.
• The number of a release's removals are the items listed when you choose a version from the dropdown.

To verify this works, here are some examples:

16.0

• In the review app:
  • 8 deprecations
  • 71 removals
• In the deprecations page:
  • 8 deprecations (search for Announced in: GitLab 16.0)
  • 71 removals (in the gitlab project: ag 'removal_milestone: "16.0"' data/deprecations | wc -l)

15.11

• In the review app:
  • 3 deprecations
  • 1 removals
• In the deprecations page:
  • 3 deprecations (search for Announced in: GitLab 15.11)
  • 1 removals (in the gitlab project: ag 'removal_milestone: "15.11"' data/deprecations | wc -l)

15.0

• In the review app:
  • 3 deprecations
  • 67 removals
• In the deprecations page:
  • 3 deprecations (search for Announced in: GitLab 15.0)
  • 67 removals (in the gitlab project: ag 'removal_milestone: "15.0"' data/deprecations | wc -l)

14.10

• In the review app:
  • 3 deprecations
  • 1 removals
• In the deprecations page:
  • 3 deprecations (search for Announced in: GitLab 14.10)
  • 1 removals (in the gitlab project: ag 'removal_milestone: "14.10"' data/deprecations | wc -l)

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
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  • 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
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  • The when to get approval handbook section explains the workflow in more detail
• 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
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