fix(gantt-chart): fix Y-axis label visibility in dark theme (#35189)

Author: LevisNgigi

superset-frontend/plugins/plugin-chart-echarts/src/Gantt/transformProps.ts

@@ -325,6 +325,7 @@ export default function transformProps(chartProps: EchartsGanttChartProps) {
           show: true,
           position: 'start',
           formatter: '{b}',
+          color: theme.colorText,
         },
         data: categoryLines,
       },

superset-frontend/plugins/plugin-chart-echarts/test/Gantt/transformProps.test.ts

@@ -257,6 +257,7 @@ describe('Gantt transformProps', () => {
           show: true,
           position: 'start',
           formatter: '{b}',
+          color: 'rgba(0,0,0,0.88)',
         },
         lineStyle: expect.objectContaining({
           color: '#00000000',