[Charts] Hover state style for partition charts and barcharts

[gvnmagni]

Problem

When hovering a serie with a light color, this is barely visible:

Compared to dark colors:

Solution

Introduce neutral colors for un-selected data series for Piecharts, Treemaps and Barcharts. All the un-selected series would become neutral while leaving the hovered one its original color.

Specifically, I would propose to use Shade 15 in light mode and Shade 110 in dark mode.

I am not entirely sure of the shade especially in light mode, it might be possible that we will need to move to higher shade (like 20) because maybe it is too light, but I'll need to see it working in the product to understand it, in Figma everything looks fine and then we need to calibrate a little

Note: the borders between bars in barcharts will be done in a follow-up issue

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[gvnmagni]

Follow-up issue for borders between elements: elastic/elastic-charts#2514