zorder property for traces causes gridline to be drawn above traces

[stephprobst]

The release of plotly.js v2.35.1 solves the problem that the zorder property didn't work correctly with overlayed subplots with multiple traces in each subplot. See #7070 for more details.

With this fix, a new problem was introduced. The gridlines of the subplots are now being influenced by the zorder of the traces, which is an unwanted side effect.

This is demonstrated in this codepen.

The blue line is drawn behind the gridlines of the the y2 axis. This should not be the case, as gridlines should always default to be drawn behind traces. Interestingly, the orange line is drawn above the gridlines, even though it is linked to the same yaxis as the blue line and also has a lower zorder than all traces on the y2 axis.

cc: @my-tien , @archmoj , @gvwilson