Updating Beta feature template for experiment features only

Why is this change being made?

Beta features currently can be both a top, primary, or secondary feature in the general section or in a dedicated beta section of the release post. This leads to confusion when reading through the release post and is updated so Beta features will always be in the general section and there is only a dedicated section for Features in Experiment.

This change updates:

• beta_features.yml to experiment_features.yml template
• Release Post rendering logic to include a section for Features in Experiment
• 16.11 release post items to use experiment release post item category instead of beta

Related Links

• Review app for 16.11 release post Features in Experiment section
• Related MR for the handbook update
• Slack discussion

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• 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
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

• Update 2 files

• /data/release_posts/unreleased/samples/beta_feature.yml
• /data/release_posts/unreleased/samples/experiment_feature.yml

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