Update handbook links to renamed auto-merge page

Why is this change being made?

In Auto-merge page updates (gitlab-org/gitlab!145872 - merged) we are moving a file:

• Old: https://docs.gitlab.com/ee/user/project/merge_requests/merge_when_pipeline_succeeds.html
• New: https://docs.gitlab.com/ee/user/project/merge_requests/auto_merge.html

This merge request updates links (both old and new) that point to this page, so they remain valid.

Related issues

• Related to Docs follow-up: Merge When Checks Pass (gitlab-org/gitlab#440696 - closed) and may even close it
• Related to Document new Merge When Checks Pass (gitlab-org/gitlab!143479 - merged) where these changes were requested but not done
• Related to #431449+
• Related to Plan for changes from 'auto_merge_labels_mr_wid... (gitlab-org/technical-writing#782)
• Related to [Feature flag] Rollout of `merge_when_checks_pass` (gitlab-org/gitlab#412995)
• Related to [Feature flag] Rollout of `additional_merge_whe... (gitlab-org/gitlab#424784 - closed)

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 links to renamed auto-merge page

This page URL is changing; let's update these old instances.

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