Add Mermaid 11 upgrade release note to 19.0 release notes

What does this MR do and why?

Adds the release note for the Mermaid version 11 upgrade to the GitLab 19.0 release notes file.

Release notes item details

• Feature Issue: #491514 (closed)
• Category: Markdown
• Tier: Free, Premium, Ultimate
• Offering: GitLab.com, GitLab Self-Managed, GitLab Dedicated
• Documentation: https://docs.gitlab.com/user/markdown/#mermaid

Checklist

• Content reviewed for accuracy
• Documentation link is valid and relative in the file
• Work item link is not confidential
• Tier and offering are correct