Upgrade mermaid to support class diagram

Author: jackycute

package.json

@@ -71,7 +71,7 @@
     "markdown-it-sup": "^1.0.0",
     "markdown-pdf": "^7.0.0",
     "mathjax": "~2.7.0",
-    "mermaid": "~6.0.0",
+    "mermaid": "~7.0.0",
     "meta-marked": "^0.4.2",
     "method-override": "^2.3.7",
     "moment": "^2.17.1",