Feat: remove deprecations from release post page

Why is this change being made?

As we no longer list deprecations on the release post page and instead provide a link to the Docs site to view them: https://docs.gitlab.com/ee/update/deprecations.html, this MR aims to remove the redundant code that was still alive in the handbook to bundle the deprecations into the release post page.

Based off this conversation: https://gitlab.slack.com/archives/C02P5KRQ1RU/p1638535856023600 where it was concluded:

If we're not listing the deprecations on the release post page anymore we can just remove that code

Related issues

#12712 (closed)

Details

After this merge two things will happen:

1. We will no longer generate a /XX_X/depreciations.yml file such as:

• 

2. We will no longer generate a list of depreciations on the XX_X release post page such as:

• 

Instead, we will show a static link to the Docs site where we list depreciations:

• 

Author Checklist

• Provided a concise title for the 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 change to the correct DRI(s)
  • 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 in 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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