Switch default renderer for class diagrams to the next generation renderer

[knsv]

The new version of class diagrams have better looks & support for htmlLabels. This will be the default renderer from now on.

It will still be able to revert to the old version by changing defaultRenderer to 'dagre-d3' in your configuration under the class section.

[mestaritonttu]

The comment still says Default value: 'dagre-d3' btw.

[ghost]

Relates to:

• Mermaid 8.0.0-rc.8 creates SVG files that do not display correctly in Inkscape and Latex mermaid-cli#103
• SVG export does not work properly when the exported SVG is used outside Firefox #2102
• Render diagrams using non-browser-based rendering libraries #2485
• SVG conformance ? #1845
• Flowchart rendered differently depending on whether "htmlLabels" is enabled or not #1572
• Move htmlLabells configuration out of flowchart to root #1431
• htmlLabels suport And CSS single Control #1395
• Label texts in flowcharts are not centered horizontally with "htmlLabels" disabled #1177
• Replace foreignObject with standard SVG #2688