[Vis colors] Improve choropleth color scales and simplify palette generation logic to reduce maintenance

[joshuarrrr]

OpenSearch Dashboards legacy map visualizations (Region Map, Coordinate map) use color schemas from truncatedColorMaps and getLegendColors(): https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/charts/public/static/color_maps/truncated_color_maps.ts#L33-L46

See opensearch-project/oui#601 for visual examples and defaults.

Instead, we should use OUI's colorPalette service to generate similar palettes with OUI color variables as the source. We can also reduce the number of color stops to 20, and simplify much of the mapping. This is very similar to #4325, but with one distinct difference - the intended design was to not have white as one end of the range, but to have even the minimum value map to a particular hue.

[manasvinibs]

Currently truncated color maps logic applies on all the color schema keys in vislibColorMaps which is Blues, Greens, Greys, Reds, yellow to Red, Green to Red.

OpenSearch-Dashboards/src/plugins/charts/public/static/color_maps/truncated_color_maps.ts

Lines 33 to 46 in 0ab3233

	export const truncatedColorMaps: ColorMap = {};
	const colormaps: ColorMap = vislibColorMaps;
	for (const key in colormaps) {
	if (colormaps.hasOwnProperty(key)) {
	// slice off lightest colors
	// @ts-ignore
	const color = colormaps[key];
	truncatedColorMaps[key] = {
	...color,
	value: color.value.slice(Math.floor(color.value.length / 4)),
	};
	}
	}

1. Logically Truncation of white color should happen only when we use single color schema such as Blue, green, greys, reds. We do not have to truncate light colors when the color gradient is moving from yellow to red/green to red.

   OpenSearch-Dashboards/src/plugins/charts/public/static/color_maps/color_maps.ts

   Line 118 in 1249d3d

   [ColorSchemas.YellowToRed]: {

2. Another thing we can do is to completely eliminate custom truncation logic here along with converting color palettes to color map on our own. Instead we can use available libraries such as D3 support to do the conversion for us.
   

   OpenSearch-Dashboards/src/plugins/charts/public/static/color_maps/color_maps.ts

   Line 68 in 1249d3d

   function convertColorPaletteToColorMap(colorPalette: string[]): RawColorSchema['value'] {