Make heatmap shades linear (#13500)

Previously, shades were 50%, 60%, 70%, 80%, 100%. Make them linearily
increasing instead at 60%, 70%, 80%, 90%, 100%.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/js/components/ActivityHeatmap.vue

@@ -28,10 +28,10 @@ export default {
     isLoading: true,
     colorRange: [
       'var(--color-secondary-alpha-70)',
-      'var(--color-primary-alpha-50)',
       'var(--color-primary-alpha-60)',
       'var(--color-primary-alpha-70)',
       'var(--color-primary-alpha-80)',
+      'var(--color-primary-alpha-90)',
       'var(--color-primary)',
     ],
     endDate: new Date(),