Various Mermaid improvements (#18776)

* Various Mermaid improvments

- Render into iframe for improved security
- Use built-in dark theme instead of color inversion
- Remove flexbox attributes, resulting in more consistent size rendering
- Update API usage and update to latest version

* restart ci

* misc tweaks

* remove unneccesary declaration

* make it work without allow-same-origin, add loading=lazy

* remove loading attribute, does not seem to work

* rename variable

* skip roundtrip to DOM for rendering

* don't guess chart height

* update comment to make it clear it's intentional

* tweak

* replace deprecated 'scrolling' property

* remove unused css file

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

package.json

@@ -23,7 +23,7 @@
     "less": "4.1.2",
     "less-loader": "10.2.0",
     "license-checker-webpack-plugin": "0.2.1",
-    "mermaid": "8.13.10",
+    "mermaid": "8.14.0",
     "mini-css-extract-plugin": "2.5.3",
     "monaco-editor": "0.32.1",
     "monaco-editor-webpack-plugin": "7.0.1",