Draft: No top, primary, secondary feature distinction

Why is this change being made?

As we move toward a fully automated release post, all features will be grouped by stage with no top, primary, or secondary distinction.

• This MR implements the changes described in https://gitlab.com/gitlab-com/Product/-/issues/5160
• Preview of 15.8

Changes

• Creates a features["all"] array from features["top"], features["primary"], and features["secondary"] in helpers/release_post_helpers.rb
• Removes %section#top-feature.top-feature and %section#primary-features.release-post-section sections from the release post template
• Loops all features as secondary format in the release post using the features["all"] array
• Updates the handbook to remove "top" and "primary" related items
• Updates spec/helpers/release_post_helpers_spec.rb to ensure all tests pass

Author Checklist

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