Draft: Write deprecations index for Release Post

What does this MR do and why?

Since 14.3, deprecations have been added to the gitlab project instead of www-gitlab-com. With that change, individual deprecations were no longer listed in the Release Post.

This MR adds a rake task that allows the Release Post Manager to create an index of deprecations in this project, which can be added to www-gitlab-com so that the Release Post can once again provide details on deprecations within the post.

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