Add custom feature_category shared

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Related gitlab-com/www-gitlab-com#14321 (comment 1259389064)

Follow-up on discussions for removing support for group::not_owned and feature_category: :not_owned.

Here is a proposal to use feature_category: :shared.

For features that support developers and they are not specific to a product group.

feature_category: not_owned support was removed with !109230 (merged)

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