Promote gitlab.MultiLineLinks to error

What does this MR do?

This promotes the gitlab.MultiLineLinks vale rule to error level to prevent new violations.

Related issues

• Related to Cleanup gitlab.MulitLineLinks Violations (#370065 - closed) where the cleanup happened in related MRs
• Merge requests proposing we change this rule from warning to error:
  • this merge request for the gitlab repo
  • Related to Promote gitlab.MultiLineLinks to error (omnibus-gitlab!6265 - merged) for omnibus-gitlab
  • Related to Promote gitlab.MultiLineLinks to error (gitlab-org/charts/gitlab!2715 - merged) for charts-gitlab
  • Related to Promote gitlab.MultiLineLinks to error (gitlab-runner!3554 - merged) for gitlab-runner

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.