[Vis colors] Replace gantt chart default series color with ouiPaletteColorBlind()[0]

[joshuarrrr]

The gannt chart visualization defines a default color: https://github.com/opensearch-project/dashboards-visualizations/blob/18771829ec0c40e18073b05632db9bfa97ebcfbb/gantt-chart/public/gantt_vis_type.ts#L118%22%3E

As part of #190, this default value should be updated to be the first color from ouiPaletteColorBlind().

see opensearch-project/oui#601 for visual comparison

[derek-ho]

Seems relatively straight forward. @ps48 assigning to you as repo owner and me as on-call but this shouldn't take too long to complete, I can take a look if you are tied up with other work

[vvavdiya]

@derek-ho @ps48
I would like to look into issue incase issue still open.