Refactor Deprecations and Breaking Changes page

Why is this change being made?

This change refactors the Deprecations, Removals and Breaking Changes section of the handbook to be its own page.

These guidelines are important information for team members, but it's currently quite difficult to follow the guide because of how it is nested within the GitLab The Product page.

This makes a next step of consolidating the advice on https://docs.gitlab.com/ee/development/deprecation_guidelines with this page easier too, as this can be a clear single-page source of truth for this information. Currently, these two pages offer overlapping advice in different ways.

It updates other links within the about.gitlab.com site to point to the new page and preserves a reference to the new page from its old location.

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, and the content is SAFE
• 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
  • 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
  • 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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