Docs: Take out removed subheading in RP link

Why is this change being made?

This release post item links to a subheading that no longer exists. The page (https://docs.gitlab.com/ee/operations/metrics/) has been revised in gitlab-org/gitlab!36886 (merged) and when the revamped version displays (within a few hours of the creation of this MR) the page will contain a large section that describes the UI of the metrics dashboard. In that section, it'll describe how to star a dashboard. As a result, we don't need the subheading any more.

Related to gitlab-org/gitlab!36886 (merged) where the need was uncovered

Related to gitlab-org/gitlab#229593 (closed) which is the follow-up to MR 36886.

Related to gitlab-org/gitlab#227441 (closed) where the overarching IA work was described.

Author Checklist

• Correct MR template applied (e.g blog post)
• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • 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.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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