Bump mermaid to 8.0.0-rc.8

This commit moves `mermaid` from blackst0ne-mermaid package to the
upstream version.

The reason why the fork was used was this bug[1].
The bug has been fixed, so there's no need to maintain my fork anymore.

[1]: https://github.com/knsv/mermaid/issues/584

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

@@ -17,7 +17,7 @@ import flash from '~/flash';
 export default function renderMermaid($els) {
   if (!$els.length) return;
 
-  import(/* webpackChunkName: 'mermaid' */ 'blackst0ne-mermaid')
+  import(/* webpackChunkName: 'mermaid' */ 'mermaid')
     .then(mermaid => {
       mermaid.initialize({
         // mermaid core options

changelogs/unreleased/blackst0ne-bump-mermaid.yml

+---
+title: Bump mermaid to 8.0.0-rc.8
+merge_request: 22509
+author: "@blackst0ne"
+type: changed

package.json

@@ -29,7 +29,6 @@
     "autosize": "^4.0.0",
     "axios": "^0.17.1",
     "babel-loader": "^8.0.4",
-    "blackst0ne-mermaid": "^7.1.0-fixed",
     "bootstrap": "4.1.1",
     "brace-expansion": "^1.1.8",
     "cache-loader": "^1.2.2",
@@ -71,6 +70,7 @@
     "jszip-utils": "^0.0.2",
     "katex": "^0.9.0",
     "marked": "^0.3.12",
+    "mermaid": "^8.0.0-rc.8",
     "monaco-editor": "^0.14.3",
     "monaco-editor-webpack-plugin": "^1.5.4",
     "mousetrap": "^1.4.6",