Update Mermaid to 8.1.0

This fixes dotted lines not rendering when the htmlLabels setting is
false (https://github.com/knsv/mermaid/pull/828).

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

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60869

app/assets/javascripts/behaviors/markdown/render_mermaid.js

@@ -59,6 +59,14 @@ export default function renderMermaid($els) {
         mermaid.init(undefined, el, id => {
           const svg = document.getElementById(id);
 
+          // As of https://github.com/knsv/mermaid/commit/57b780a0d,
+          // Mermaid will make two init callbacks:one to initialize the
+          // flow charts, and another to initialize the Gannt charts.
+          // Guard against an error caused by double initialization.
+          if (svg.classList.contains('mermaid')) {
+            return;
+          }
+
           svg.classList.add('mermaid');
 
           // pre > code > svg

changelogs/unreleased/sh-update-mermaid.yml

+---
+title: Update Mermaid to 8.1.0
+merge_request: 30036
+author:
+type: fixed

package.json

@@ -96,7 +96,7 @@
     "jszip-utils": "^0.0.2",
     "katex": "^0.10.0",
     "marked": "^0.3.12",
-    "mermaid": "^8.0.0",
+    "mermaid": "^8.1.0",
     "monaco-editor": "^0.15.6",
     "monaco-editor-webpack-plugin": "^1.7.0",
     "mousetrap": "^1.4.6",