Update Mermaid to 8.1.0

This fixes dotted lines not rendering when the htmlLabels setting is false.

Full list of changes: https://github.com/knsv/mermaid/releases

Closes:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/60869 (via https://github.com/knsv/mermaid/pull/828)
• https://gitlab.com/gitlab-org/gitlab-ce/issues/50459 (via https://github.com/knsv/mermaid/pull/791)